Electrochemical properties of niobium and phosphate doped spherical Li-rich spinel LiMn2O4 synthesized by ion implantation method

Article abstract of DOI:10.1016/j.cclet.2017.03.035

Spherical Li-rich lithium manganese oxide (LMO) spinel material was synthesized by an ion implanted method assisted by polyalcohol doped with Niobium and Phosphate simultaneously. The material was characterized by scanning electron microscopy, X-ray diffraction and BET specific surface area analysis. The electrochemical performances were investigated with galvanostatic techniques and cyclic voltammetry. The synthesis process was investigated with TG/DSC. The results show that the lithium ion can be immersed into the pore of manganese dioxide at a low temperature with the ion implanted method. The prepared materials have a higher discharge capacity and better crystallization than those prepared by solid phase method. The doped Nb can improve the capacity of the Li-rich LMO spinel and reinforce the crystal growth along (111) and (400) planes. The crystal grains show circular and smooth morphology, which makes the specific surface area greatly decreased. Phosphate-doped LMO spinel exhibits good high-rate capacity and structure stability. The prepared Li_1.09Mn_1.87Nb_0.031O_3.99(PO₄)_0.021 delivers a discharge capacity of 119?mAh?g^?1 at 0.2?°C (1?°C?=?148?mA?g^?1) and 112.8?mAh?g^?1 at 10?°C, the discharge capacity retention reaches 98% at 1?°C after 50 cycles at 25?°C and 94% at 55?°C.

Full text of DOI:10.1016/j.cclet.2017.03.035

Accepted Manuscript
Title: Electrochemical properties of niobium and phosphate
doped spherical Li-rich spinel LiMn2O4 synthesized by ion
implantation method
Authors: Wei Li, Gao-Wa Siqin, Zhi Zhu, Lu Qi, Wen-Huai
Tian
PII:
S1001-8417(17)30123-7
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.cclet.2017.03.035
CCLET 4030
To appear in:
Chinese Chemical Letters
Received date:
Revised date:
Accepted date:
20-12-2016
7-2-2017
26-3-2017
Please cite this article as: Wei Li, Gao-Wa Siqin, Zhi Zhu, Lu Qi, Wen-Huai
Tian, Electrochemical properties of niobium and phosphate doped spherical Li-
rich spinel LiMn2O4 synthesized by ion implantation method, Chinese Chemical
Lettershttp://dx.doi.org/10.1016/j.cclet.2017.03.035
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Original article
Electrochemical properties of niobium and phosphate doped spherical Li-rich spinel
LiMn2O4 synthesized by ion implantation method
a,
a
b
b
a

Wei Li , Gao-Wa Siqin , Zhi Zhu , Lu Qi , Wen-Huai Tian
a Department of Materials Physics and Chemistry, School of Materials Science and Engineering, University of Science and Technology, Beijing 100083, China
b New Energy Materials and Technology Laboratory, Department of Applied Chemistry, College of Chemistry and Molecular Engineering, Peking University,
Beijing 100871, China.
 Corresponding author.
E-mail address: liweimgl@163.com
ABSTRACT.
Spherical Li-rich lithium manganese oxide (LMO) spinel material was synthesized by an ion implanted method assisted by polyalcohol doped with
Niobium and Phosphate simultaneously. The material was characterized by scanning electron microscopy, X-ray diffraction and BET specific surface
area analysis. The electrochemical performances were investigated with galvanostatic techniques and cyclic voltammetry. The synthesis process was
investigated with TG/DSC. The results show that the lithium ion can be immersed into the pore of manganese dioxide at a low temperature with the
ion implanted method. The prepared materials have a higher discharge capacity and better crystallization than those prepared by solid phase method.
The doped Nb can improve the capacity of the Li-rich LMO spinel and reinforce the crystal growth along (111) and (400) planes. The crystal grains
show circular and smooth morphology, which makes the specific surface area greatly decreased. Phosphate-doped LMO spinel exhibits good high-
rate capacity and structure stability. The prepared Li1.09Mn1.87Nb0.031O3.99(PO4)0.021 delivers a discharge capacity of 119 mAh g-1 at 0.2 C (1 C=148
mA g-1) and 112.8 mAh g-1 at 10 C, the discharge capacity retention reaches 98% at 1 C after 50 cycles at 25 °C and 94% at 55 °C.
Keywords:
Spinels
Ion implantation
Li-ion batteries
LiMn2O4
Doping
Article history:
Received 21 December 2016
Received in revised form 7 February 2017
Accepted 14 March 2017
Available online
.
Graphical Abstract
———

Spherical Li-rich lithium manganese oxide spinel material was synthesized by an ion implanted method assisted by polyalcohol
doped with Niobium and Phosphate simultaneously. The prepared Li1.09Mn1.87Nb0.031O3.99(PO4)0.021 delivers a discharge capacity
of 119 mAh g-1 at 0.2 C (1 C=148 mA g-1) and 112.8 mAh g-1 at 10 C, the discharge capacity retention reaches 98% at 1 C
after 50 cycles at 25 °C and 94% at 55 °C.
1. Introduction.
Lithium manganese oxide (LMO) spinel has been considered as one of the most promising cathode materials for large power
batteries and energy-storage systems in recent years. It exhibits good electrochemical performance and high safety at high current rate.
Manganese is one of the most abundant elements in the Earth crust, and it is environmentally benign. However, this LMO material
shows a significant capacity fading during long-term electrochemical cycling, particularly at elevated temperatures (55 °C), mainly
because of the following possible reasons: (1) the structural collapse due to Jahn-Teller distortion in the charge and discharge process,
(2) enhanced manganese ion dissolution into the electrolyte, (3) the thermal decomposition of LiPF6-based carbonate electrolytes [1-5].
A large number of efforts are carried out to resolve the above issues. First of all, synthesis methods are studied. The traditional
methods mainly include solid state method and soft chemical method. For solid state method, repeated grinding and high temperature
reaction for a long time are always needed to form spinel crystal. Soft chemical methods include Pechini method, sol-gel method,
hydrothermal method and co-precipitation method etc. [6-12]. In these methods, the precursors are obtained from liquid solution, so all
the components can be distributed very homogeneously at atomic-molecular level in these precursors. Soft chemical synthesis can be
performed at comparatively low temperatures and within short periods. Solid state method is an easy preparation process, the obtained
product shows high bulk density but poor electrochemical performance. In the synthesis process, some undesirable impure phases can
be formed in the product because of the long-range diffusion of atoms. The product synthesized by soft chemical method shows good
electrochemical performance but low density, and it is a complex process. In recent years, some new method are put forward such as
solid-state combustion method [13], electrospinning method [14], spray pyrolysis method [15], solution combustion method [16] and
mechanical alloying method [17].
Secondly, modified methods are adopted to improve the performance of LMO spinel including doping and coating. The investigated
doping ions include cations and anions, the cationic doping elements mainly include Li, Mg, Ni, Zn, Cu, Al, Cr, Co, Ga, Gd, La, Ce,
Nd, Sm, Tb, Sr, Dy, Mo, B, Fe, Ti, Sn [18-30], Er, Sc, Y [2] etc., while the reported anions mainly consist of F- [31], Cl-, Br- [2], S2-
3-
3-
2-
[32], BO3 ,PO4 , SiO3 [33,34] etc.. Sometimes, single ion doping cannot reach satisfactory effect, dual or multiple ions doping
simultaneously have also been developed. Surface coating is an effective approach in which an artificial protective layer is used to
prevent Mn ions from contacting with electrolyte, and restrain the Mn dissolution. The coating materials mainly include various oxides,
fluorides, phosphates, lithium compounds, carbon materials, and polymers [2, 35].
Besides doping and coating, some new techniques are developed in LMO spinel such as controlling the nanoparticles granularity and
spheroidizing of particles [2, 13, 16, 36]. Although a certain progress has been made in these researches, LMO spinel has not been put
into large-scale commercial applications. The properties of LMO spinel need to be further improved.
In terms of choice for doping elements, few researchers select +5 valence cations. L. Hernan et al. investigated the electrochemical
properties of vanadium doped LMO spinel in different voltage window [37]. The results show that it has a discharge capacity of 105
Ah kg-1 within the range of 4.6-2.5 V. Herein, we synthesized a spherical Li-rich LMO spinel cathode material doped with +5 valence
niobium ion and phosphate by an ion implanted method. The morphology of the prepared material is obtained by controlling the
morphology of the manganese dioxide precursors. The properties of the material and synthesis mechanism were also investigated in
detail in this work.
2. Results and discussion.
2.1. XRD analysis on the raw materials, MnO2 precursor, SP and IP
Fig.1 and Fig.S1 (Supporting Information) are the XRD patterns of the precursor mixtures prepared by different method and the raw
material lithium hydroxide monohydrate and manganese dioxide precursor. Fig.S1a (Supporting Information) suggests that the lithium
hydroxide monohydrate is regular crystal structure with high crystallinity, while the precursor MnO2 indicate a γ-MnO2 structure
(JCPDS card No. 30-820) with low crystallinity as shown in Fig.S1b (Supporting Information) [36, 38].
Fig.1 shows that the XRD patterns of SP comprise the patters of lithium hydroxide monohydrate and manganese dioxide. The
characteristic peaks of lithium hydroxide monohydrate and manganese dioxide both appear on the diffraction pattern of SP. It
demonstrates that the lithium hydroxide monohydrate and the manganese dioxide were just simply blended in physical form. However,
the characteristic peaks of the lithium hydroxide monohydrate disappear in the XRD patterns of IP. It implies that lithium hydroxide
monohydrate has been dissolved.
2.2. Thermal analysis
To further investigate the component of the mixture and synthesis process, TG/DSC is carried out from room temperature to 900 °C.
Fig.2 (a) is the TG-DSC curve of the pure lithium hydroxide monohydrate. A dehydration process can be found at 100 °C with a strong
endothermic peak in DSC curve accompanied by a significant weight loss in TG curve. The solid lithium hydroxide transforms into

liquid phase at 430-480 °C where there is a strong heat absorption phenomenon without weight change. Lithium hydroxide decompose
at about 600 °C with weight loss. The whole thermal reaction process can be revealed by Eq. (1) - (3):
LiOH·H2O→ LiOH+ H2O
2LiOH(s)→2LiOH(l)
2LiOH(l)→ Li2O+H2O
(1)
(2)
(3)
In this experiment, 43.0% of weight loss is observed, and it is extremely close to the theoretical weight loss value of 42.8% calculated
from Eq. (1) - Eq. (3).
Fig.2 (b) is the curve of manganese dioxide precursor. The results show that there is a small amount of heat absorption along with
continuous weight change. It should be related with the adsorbed water removal from the manganese dioxide. It can be inferred that the
endothermic decomposition of the manganese dioxide happened near 555 °C because of the apparent sharp endothermic peak with a
straight line loss in weight. The reaction is as follows:
4MnO2→2Mn2O3+O2
(4)
Fig.2(c) is the TG-DSC curve of the precursor SP obtained by the solid method. It can be found that there are five endothermic peaks
corresponding to the dehydration of lithium hydroxide monohydrate, the melting of lithium hydroxide, the decomposition of lithium
hydroxide, the decomposition of manganese dioxide and the forming of LMO spinel respectively at 100 °C, 400 °C, 500 °C, 650 °C,
830 °C. It may exist the following reaction equations according to the raw material component of SP.
LiOH·H2O→ LiOH+ H2O
LiOH(S) →LiOH(L)
2LiOH→Li2O+ H2O
4MnO2→2Mn2O3+ O2
Li2O+2Mn2O3+1/2O2→ 2Li Mn2O4
(5)
(6)
(7)
(8)
(9)
It is obvious that the endothermic peaks are consistent with that of the pure lithium hydroxide and manganese dioxide except the
peak at 830 °C, although some alterations to the temperature coordinates may be caused by the instrument. It also proves that SP is
simply consisted of lithium hydroxide and manganese dioxide by physical mixing. The results are consistent with that of XRD
analysis. The Eq. (9) is not so consistent with the TG curve at about 830 °C, it indicates that the formation process of the LMO spinel
is complex in the solid state method. The theoretical weight loss is 19.2% according to Eq. (5)-(9), while the actual experimental result
is 24.6%. It is reasonable because the manganese dioxide usually contains about 5 wt% water.
Fig.2 (d) is the TG-DSC curve of the precursor IP obtained by the ion implanted method. It shows that there are two endothermic
peaks on the DSC curve at about 560 °C and 800 °C along with two significant weight change on TG curve. The two peaks are
corresponding to the decomposition of manganese dioxide and the formation of LMO spinel. It can also be inferred that there is no
lithium hydroxide in IP, because no peaks due to the dehydration of lithium hydroxide monohydrate and the melting of lithium
hydroxide were found. The possible reaction mechanism is as follows, lithium hydroxide monohydrate is dissolved with the help of
ethylene glycol or glycerol in the mixing process, when a large amount of heat is produced, and the lithium ions have been implanted
into the pore of manganese dioxide under the action of the heat. Then the mixture is transformed into Li2O·4MnO2 compound after
drying. So the forming process of LMO can be represented by Eq. (10).
Li2O·4MnO2→ 2LiMn2O4+1/2O2
(10)
The actual experimental weight loss is 5.0%. It is close to the theoretical value of 4.2% according to Eq. (10).
2.3. The morphology analysis
Fig.3 (a) is the SEM image of the MnO2 precursor. It shows that the pristine submicron fibrous MnO2 particles were gathered into
micrometer spherical secondary particles. A large number of pores were formed between the fibrous particles, which would provide
good channels for the lithium ion injection in the ion implantation method. The spherical particle diameter is of about 10 microns.
Fig.3 (b)-(f) are the SEM images of the prepared LMO. All the particles with spherical morphology were consisted of primary
submicron crystalline grain with different size and shape. SLM is consisted of the octahedral and irregular tiny particles. It indicates
that the material does not fully grow into octahedral particles during the high temperature reaction. The particles of LM and LLM are
composed of octahedral grains with high crystallization, while those of LLMN and LLMNP are composed of grains with irregular
shape and smooth surface. Besides, the octahedral crystal edges disappear in LLMN and LLMNP.
The results of Fig. 3 demonstrate that the synthesis method and doping have influence on the particle morphology of lithium
manganese oxide spinel prepared. This effect will be further reflected from the microstructure and electrochemical performance.
2.4. XRD analysis of as prepared samples
Fig.4 is the XRD patterns of the prepared samples in the range of 2θ from 10-90°. The diffraction peaks of the samples can be well
indexed to a cubic spinel structure with Fd-3m space group, which is consistent with JCPDS Card No: 35-0782. The diffraction peak
intensity of SLM is lower than that of the other samples. There are two weak diffraction peaks at ca. 32.5° and 55.3° for SLM which
corresponds to the strongest characteristic diffraction peak of Mn2O3 (PDF 78–0390), indicating the presence of a trace amount of
Mn2O3 impurity phase in this material [39]. It cannot be noticed any strange reflection in the patterns of LM, LLM, LLMN, LLMNP.
The (220) peak cannot be found in the XRD patterns for all the five samples. It indicates that the tetrahedral sites (8a) are only
occupied by lithium atoms.
The ratios of the diffraction peak intensity due to different crystal planes and corresponding 2θ are showed in Table S1(Supporting
information). The ratios I(111)/ I(311) of sample SLM, LM, LLM, LLMN, LLMNP are 2.68, 2.69, 2.87, 2.03, 2.15 and 2.53, 3.16, 2.53,
2.34, 2.24 for I(111) /I(400) respectively. The ratios of I(111)/I(311) and I(111) /I(400) for sample SLM, LM, LLM are larger than that of LLMN,
LLMNP, it indicates that SLM, LM, LLM crystal particles grow well along the (111) crystal plane, so the particles exhibit an

octahedral shape. The ratios of I(111)/I(311) and I(111)/I(400) for sample LLMN and LLMNP decrease, which implies that the crystal growth
along the (311) and (400) surface is enhanced, while that along the (111) surface is restrained. So the crystalline grain presents arc
shape with smooth surface without crystal edge. This result is consistent with the SEM results shown in Fig. 3.
The location and strength of the diffraction peaks and lattice parameters are also summarized in Table S1 (Supporting information).
After lithium ions have replaced parts of the manganese ions in LLM, the lattice parameter decreases from 8.2417 Å(LM) to 8.2296
Å(LLM). The diffraction peaks are slightly shifted to larger angle as shown in Fig. S2a-c (Supporting information). It may be caused
by the reason that the content of the Mn4+ is increased because of the replacement of the Mn with Li, while the radius of Mn4+ ion is
smaller than that of Mn3+ ion [40]. The lattice parameter of LLMN (8.2363 Å) is increased compared to LLM, because Nb5+ radius is
close to Mn3+, and the Mn3+ content is much increased after doping niobium. The lattice parameter of LLMNP (8.2394 Å) is increased
compared to LLMN, for the radius of polyanion phosphoric acid root is larger than that of oxygen ionic. All in all, the lattice parameter
is increased after doping with niobium and phosphate ions and the diffraction peaks move toward to smaller angle as shown in Fig.
S2a-c (Supporting information).
2.5. The chemical composition analysis
The XPS was carried out to determine the average oxidation state of manganese. Fig.5 represents the Mn 2p XPS spectra for as
prepared samples, in comparison with those for MnO2 and Mn2O3. The Mn 2p3/2 peak of MnO2 is observed at 642.6 eV and that of
Mn2O3 at 641.6 eV. As shown in Fig.5, the Mn 2p3/2 peaks of as prepared samples are observed between those of MnO2 and Mn2O3.The
Mn 2p3/2 peak position of LLM, LLMN and LLMNP shifts toward higher binding energies, it indicates that the average oxidation state
of manganese in the three samples is increased. The average Mn valence and the relative ratio of Mn3+ and Mn4+ are calculated based
on the results of XPS and summarized in Table S2 (Supporting information).
The contents of Li, Mn, Nb and P were measured using ICP-AES. The O content was calculated based on the ICP-AES data and the
average Mn valence. The chemical composition of the as-prepared samples was shown in Table 2 (Supporting information). The actual
chemical formulas are very close to the design formulas.
2.6. Electrochemical performance analysis
Fig. 6 is the initial charge and discharge curves of SLM, LM, LLM, LLMN and LLMNP at 0.2 C (1 C=148 mA g-1) rate. The
discharge specific capacities are 113 mAh g-1, 123 mAh g-1, 117 mAh g-1, 120 mAh g-1 and 119 mAh g-1 respectively. The specific
discharge capacity of the sample synthesized by solid method is lower than that by the ion implantation method. This is mainly related
to the different crystallization of the product prepared by the two methods. The sample synthesized by the solid method has a poor
crystallization and contains impurities, while the samples synthesized by the ion implantation method have a good crystallization.
Fig. 6 also shows that the doping has great influence on the capacity. In the process of charging and discharging, the occurred redox
reaction is mainly: Mn3+ → Mn4+. Thus, the capacity is determined by Mn3+ content [41]. Compared with LM, excess lithium ions were
doped in LLM, so that part of Mn3+ would become Mn4+ and the average manganese valence would rise from 3.530 to 3.640 as shown
in Table S2 in Supporting information. The specific discharge capacity of LLM is 117 mAh g-1, while that of normal spinel LM is 123
mAh g-1. The specific discharge capacity of LLMN recovers to 120 mAh g-1 for the Mn3+ content is increased after doping with
pentavalent niobium ion. When doping phosphoric acid root, Mn3+ concentration is slightly decreased, thus the discharge capacity of
LLMNP is declined to 119 mAh g-1.
The two voltage plateaus in 4 V region are obvious in sample SLM and LM, but not so evident in sample LLM. It is found that the
voltage jump disappeared in sample LLMN and LLMNP and the two voltage plateaus do not exist. This suggests that the two-phase
structure is transformed to an one-phase structure in the charging and discharging process [15].
Fig. 7 is the cycle performance of the prepared samples in room temperature (25 °C) and high temperature (55 °C) respectively at 1
C rate. The capacity retention rates of SLM, LM, LLM, LLMN and LLMNP are 87.5%, 87.6%, 95.4%, 94.4% and 97.0% respectively
after 50 cycles at 25 °C and 83.7%, 84.1%, 90.3%, 92.1% and 92.0% at 55 °C. It indicates that doping can improve the cycle
characteristics at both room temperature and high temperature. The sample LLMN and LLMNP doped with niobium have better cycle
characteristics than LLM doped with single lithium at high temperature. The coulombic efficiency of these samples is showed in Fig.
S3(Supporting information) during the process of cycling. It also indicates that the sample LLMN and LLMNP have a higher
coulombic efficiency than the other samples.
In spinel structure, 32 oxide ions form a face centered cubic lattice, and the lithium and manganese ions occupy 8a tetrahedral sites
and 16d octahedral sites, respectively [35]. When doping excess lithium ions, part of the lithium ions would replace the manganese
ions in 16d octahedral sites, which leads to the content of Mn4+ ions increased to keep the charge balance. Because the radius of Mn4+
ion is smaller than that of Mn3+ ion, the lattice constant would decrease, and the manganese oxide covalent bond strength would also be
increased, so the crystal structure became more stable. Doretta Capsoni reported that the capability of a lattice to maintain the
undistorted structure is determined by the ratio r = [Mn 4+]/[Mn 3+] [42]. Jahn–Teller transition can be suppressed when r≥1.18. As
shown in Table S2 (Supporting information), the r values of LLM, LLMN and LLMNP reach 1.78, 1.56 and 1.63. After doping with
niobium and phosphorus, although the crystal structure constant slightly increases, but the r value is greater than 1.18, so LLMN and
LLMNP still have good cycle characteristics. Q. Luo [20] reported that the Mn valence in spinel cathodes needs to be >3.6 to offer
good cycle ability at 25 °C and 55 °C. The average valences of manganese are 3.522, 3.530, 3.640, 3.610 and 3.625 as shown in Table
S2 (Supporting information). The experimental results are consistent with the conclusion.
The cycling features of LLMN and LLMNP are better than that of LLM at high temperature. The reasons may be associated with the
morphology of circular arc and smooth surface due to the niobium dopant, which can reduce the contact area between electrode and

electrolyte due to the small specific surface area as shown in Table S3 (Supporting information). It is well known that the capacity
retention of LMO spinel would typically deteriorate at elevated temperature which is related to the manganese dissolution [31].
Fig.7 indicates that the sample LLMNP with phosphate ion dopant exhibits the best cycle performance. In this case, the Mn-O
distance is increased due to the phosphate doping, thus the polarizing effect exerted by Mn3+ over O2- is decreased, and the positive
charge on the Mn atom is increased as well. It produces a beneficial effect that the average valence of manganese would increase when
increasing the a-lattice parameter. This effect can be used strategically to diminish the influence of the undesired Jahn-Teller effect on
the electrochemical behavior of the LiMn2O4 cathode material through phosphate doping [43]. The electrochemical performance of the
optimal sample and some published results of Li-rich spinels are showed in Table S4 (Supporting information).
Fig. 8 represents the rate capability of the as-prepared samples at different rates ranging from 0.2 C to 10 C and the recovery ability
after the charging and discharging at high rate. It shows that the discharge capacities decrease with the increasing charge–discharge
rate. The specific discharge capacity of SLM, LM, LLM, LLMN and LLMNP are 113 mAh g-1, 123 mAh g-1, 117 mAh g-1, 120 mAh g-
1
and 119 mAh g-1 at 0.2 C rate, 106.6 mAh g-1, 116.2 mAh g-1, 109.2 mAh g-1, 115.4 mAh g-1 and 118.7 mAh g-1 at 1 C rate, 102.5
mAh g-1, 112.4 mAh g-1, 105.9 mAh g-1, 111.6 mAh g-1 and 114.6 mAh g-1 at 5 C rate, which then decrease to 96 mAh g-1, 105 mAh g-
1, 98.5 mAh g-1, 108.3 mAh g-1and 112.8 mAh g-1 at 10 C rate respectively. When the charge-discharge rates reduce from 10 C to 0.2 C,
the specific discharge capacity of these samples could be recovered with 2.9%, 2.2%, 2.7%, 1.0%, 0.5% capacity loss. The two
samples doped with niobium have more superior rates characteristics than those of the other samples. Doping the phosphate root
further improves the rate characteristics. LLMNP has the best cycle characteristics at high rates.
The cyclic voltammograms of the prepared samples are shown in Fig. S4 (Supporting information) at a cutoff voltage between 3.0 V
and 4.35 V with a scanning rate of 0.2 mV s-1. The CV curve of the un-doped samples SLM and LM shows two sharp anodic and
cathodic peaks, it is corresponding to the two voltage plateaus of the charge and discharge curve as shown in Fig.6. There are two
binary equilibrium systems (LiMn2O4-Li0.5Mn2O4 and Li0.5Mn2O4-λMnO2) for normal spinel LiMn2O4 during Li ion intercalation and
deintercalation with a 100 mV voltage difference. The first pair redox of the peaks (4.15 V/4.05 V) is attributed to the intercalation/de-
intercalation of lithium ion into the first half of the tetrahedral sites, where Li+ ions do not interact. The second pair of the peaks (about
4.05 V/3.95 V) is attributed to the removal/addition of Li ion from/into the other half of tetrahedral sites in which Li/Li+ interaction
occurs. It would increase the repulsion between the lithium ions, resulting in a small increase in the free energy in the system as
reflected by the observed potential dropping or rising [7, 10]. It can be inferred that the change of structure is rapid in the process of
charging and discharging for the un-doped samples SLM and LM, it would lead to final structure damage and deteriorate the cyclic
performance. Fig. S4 (Supporting information) also shows that the sample LM has better reversibility than SLM.
The CV of the Li-rich spinel LLM shows two gentle redox peaks. The first pair redox of the peaks (4.15 V/4.05 V) becomes slight
bulge for the CVs of LLMN while the two peaks disappear and become one peak for the CVs of LLMNP. It indicates that the structure
changes gradually in the process of charging and discharging due to the adding of dopant. It helps to improve the cycle characteristics
of LLM, LLMN and LLMNP.
Fig. S4 (Supporting information) also shows that the sample LLMNP has the largest peak current compared with that of other
samples at the same scanning rate, so it can be inferred that the sample LLMNP has the highest Li+ ion diffusion coefficient which
accounts for an excellent rate capability.
3. Conclusion.
Spherical LMO spinel was prepared by an ion implanted method assisted by polyalcohol. Compared with the solid phase method,
lithium ion can be immersed into the pore of manganese dioxide at a low temperature by the ion implantation method. The materials
prepared by the ion implantation method have a high discharge capacity and excellent crystallization.
The LMO material was doped with lithium, niobium, and phosphate simultaneously. The results show that the rich lithium doping
can improve the cycle stability at both room temperature and high temperature. Niobium doping can enhance the capacity of Li-rich
LMO spinel. The crystal growth along the (111) and (400) planes is reinforced. The crystal grain exhibits circular shape and smooth
morphology, so the BET specific surface area is decreased. Phosphate-doped LMO spinel exhibits good high-rate capacity and
structure stability. The prepared Li1.09Mn1.87Nb0.031O3.99(PO4)0.021 has a discharge capacity of 119 mAh g-1 at 0.2 C rate and 112.8 mAh
g-1 at 10 C rate. The retention rates of the discharge capacity reach 98% at 1 C rate after 50 cycles in 25 °C and 94% in 55 °C.
4. Experimental.
All the chemical reagents are of analytically pure grade. Reagents and materials used for the electrochemical performance evaluation
are specialized for lithium ion battery.
4.1. Synthesis of spherical MnO2 precursor
To prepare spherical MnO2, a redox reaction was adopted in a liquid phase using sodium permanganate solution and manganese
chloride as reactant. Firstly, the manganese chloride was dissolved in deionized water with the concentration of 1.5 mol L-1 and the
sodium permanganate solution was diluted to 1.5 mol L-1. Then some H2SO4 solution with pH=5 was added into a reaction kettle which
was placed in a 40 °C water bath pot. After that, the sodium permanganate and manganese chloride solution were added into the
reaction kettle continuously with a feeding speed of 8 mL min-1 by constant flow pump under the condition of stirring. Keep stirring for
half an hour after the end of feeding, the mixture was aged for ten hours. Finally, the spherical MnO2 precursor was obtained by
filtering, washing and drying. The Mn wt% was measured by inductively coupled plasma atomic emission spectroscopy.
4.2. Synthesis of LiMn2O4 sample by a solid state reaction method

A stoichiometric mixture of lithium hydroxide monohydrate (LiOH·H2O) and spherical MnO2 according to the formula LiMn2O4
was thoroughly mixed by grinding in agate mortar for two hours. The mixture (denoted by SP) then sintered at 800 °C for 6 hours in
air, followed by slow cooling to ambient temperature. The LiMn2O4 sample synthesized by this solid state reaction was marked as
SLM.
4.3. Synthesis of LiMn2O4, Li1.1Mn1.9O4, Li1.1Mn1.87Nb0.03O4, Li1.1Mn1.87Nb0.03O3.98(PO4)0.02 samples by an ion implanted method.
Lithium hydroxide monohydrate (LiOH·H2O) and spherical MnO2 were mixed according to the stoichiometric ratio in LiMn2O4,
then 20 wt% ethylene glycol or glycerol was added into the mixture as soluble initiator, lithium hydroxide monohydrate was
dehydrated and dissolved with the help of ethylene glycol or glycerol. Meanwhile, lithium ions were implanted into the pore of
manganese dioxide under the action of solution heat. Then, a mixed paste was attained. After drying at 80 °C for 20 hours the mixed
paste turned into mixture powder (denoted by IP). The sample IP was calcined at 800 °C for 6 hours in air atmosphere. The final
sample LiMn2O4 was marked as LM. Li1.1Mn1.9O4 (denoted as LLM) was synthesized by the same method. Using niobium pentoxide,
diammonium phosphate as dopant, Li1.1Mn1.87Nb0.03O4 (denoted by LLMN) and Li1.1Mn1.87Nb0.03O3.98(PO4)0.02 (denoted by LLMNP)
were synthesized by the same method.
4.4. Characterization of the as-prepared samples
The phase identification of the prepared samples was carried out by X-ray diffraction (XRD) using a Multiflex X-ray powder
diffractometer. X-ray profiles were measured between 10 and 90° (2θ angle) with a monochromatic Cu radiation source. The
morphologies of the samples were examined with a JEOLJSM-5600LV scanning electron-microscope (SEM). Thermal analysis was
carried out with a TGA/DSC (Setaram Labsys) instrument at a heating rate of 10 °C min-1. The BET specific surface area was
measured by N2 sorption using surface analyzer Gemini 2360. Elemental analysis was carried out using inductively coupled plasma
atomic emission spectroscopy (ICP-AES) (PROFILE SPEC, USA) to determine the chemical composition of the prepared samples.
The present XPS data were collected using a Imaging X-ray Photoelectron spectrometer (Axis Ultra, UK) with a monochromatized Al
Kα source.
The electrochemical properties of the samples were tested in CR2032 coin-type cells with metallic lithium as anode electrode.
Cathode slurries were prepared by mixing 90 wt% spinel LMO, 4 wt% conducting carbon black, and 6 wt% PVDF binder in NMP
solvent. Keep stirring for two hours by magnetic force, these slurries were then cast onto an aluminum foil and dried at 120 °C in a
vacuum oven. Then the foil was punched into circular electrode. The cathode was separated from the Li anode by a layer of celgard
2300 membrane soaked with the electrolyte of 1 mol/L LiPF6 in a 1:1(v/v) mixture of ethylene carbonate (EC) and dimethyl carbonate
(DMC). The cells were assembled in an argon-filled glove box. The charge and discharge curves, cycle performances and rate
characteristics of the cells were evaluated on a Land cell tester. Cyclic voltammograms (CVs) were measured with CHI660C type
electrochemical workstation.
Acknowledgment
This work was supported by a grant from the National High Technology Research and Development Program of China (863
Program) (No. 2008AA11A102).
References
[1] N. Nitta, F.X. Wu, J.T. Lee, et al., Li-ion battery materials: present and future, Mater. Today18 (2015) 252-264.
[2] G.J. Xu, Z.H. Liu, C.J. Zhang, et al., Strategies for improving the cyclability and thermo-stability of LiMn2O4-based batteries at elevated temperatures, J.
Mater. Chem. A 3 (2015) 4092-4123.
[3] S. Martinez, I. Sobrados, D. Tonti, et al., Chemical vs. electrochemical extraction of lithium from the Li-excess Li1.10Mn1.90O4 spinel followed by NMR and
DRX techniques, Phys. Chem. Chem. Phys. 16 (2014) 3282-3291.
[4] Y.F. Deng, S.X. Zhao, D.H. Hu, et al., Structure and electrochemical performance of spinel LiMn1.95Ni0.05O3.98F0.02 coated with Li-La-Zr-O solid electrolyte,
J. Solid State Electrochem. 18 (2014) 249-255.
[5] Z. Zhu, D. Zhang, H. Yan, et al., Precise preparation of high performance spherical hierarchical LiNi0.5Mn1.5O4 for 5 V lithium ion secondary batteries, J.
Mater. Chem. A 1 (2013) 5492-5496.
[6] T. Nakamura, A. Kajiyama, Synthesis of Li–Mn spinel oxide using Mn2O3 particles, Solid State Ionics 124 (1999) 45-52.
[7] W. Liu, K. Kowal, G.C. Farrington, Electrochemical Characteristics of Spinel Phase LiMn2O4-Based Cathode Materials Prepared by the Pechini Process, J.
Electrochem. Soc. 143 (1996) 3590-3596.
[8] L.J. Fu, H. Liu, C. Li, et al., Electrode materials for lithium secondary batteries prepared by sol–gel methods, Prog. Mater. Sci. 50 (2005) 881-928.
[9] T. Kanasaku, K. Amezawa, N. Yamamoto, Hydrothermal synthesis and electrochemical properties of Li–Mn-spinel, Solid State Ionics 133 (2000) 51-56.
[10] Y.L. Cui, W.J. Bao, Z. Yuan, et al., Comparison of different soft chemical routes synthesis of submicro-LiMn2O4 and their influence on its electrochemical
properties, J. Solid State Electrochem. 16 (2012) 1551-1559.
[11] C.Y. Zhu, G. Saito, T. Akiyama, A new CaCO3-template method to synthesize nanoporous manganese oxide hollow structures and their transformation to
high-performance LiMn2O4 cathodes for lithium-ion batteries, J. Mater. Chem. A 1 (2013) 7077-7082.
[12] Z. Zhu, L. Qi, D. Zhang, et al., Preparation of spherical hierarchical LiNi0.5Mn1.5O4 with high electrochemical performances by a novel composite co-
precipitation method for 5 V lithium ion secondary batteries, Electrochim. Acta 115 (2014) 290-296.
[13] Y.J. Cai, Y.D. Huang, X.C. Wang, et al., Facile synthesis of LiMn2O4 octahedral nanoparticles as cathode materials for high capacity lithium ion batteries
with long cycle life, J. Power Sources 278 (2015) 574-581.
[14] X.N. Ding, H.W. Zhou, G.C. Liu, et al., Electrochemical evaluation of LiAl0.05Ni0.05Mn1.9O4 cathode material synthesized via electrospinning method, J.
Alloys Comp. 632 (2015) 147-151.
[15] S. Hirose, T. Kodera, T. Ogihara, Synthesis and electrochemical properties of Li-rich spinel type LiMn2O4 powders by spray pyrolysis using aqueous
solution of manganese carbonate, J. Alloys Comp. 506 (2010) 883-887.
[16] Q.T. Zhang, J.T. Mei, X.M. Wang, et al. Facile synthesis of spherical spinel LiMn2O4 nanoparticles via solution combustion synthesis by controlling
calcinating temperature, J. Alloys Comp. 617 (2014) 326-331.
[17] T.Y.S.P. Putra, M. Yonemura, S. Torii, et al., Structure and electrochemical performance of the spinel-LiMn2O4 synthesized by mechanical alloying, Solid

State Ionics 262 (2014) 83-87.
[18] M. Saitoh, M. Sano, M. Fujita, et al., Studies of Capacity Losses in Cycles and Storages for a Li1.1Mn1.9O4 Positive Electrode, J. Electrochem. Soc. 151
(2004) A17-A22.
[19] E. Talik, L. Lipińska, P. Zajdel, et al., Electronic structure and magnetic properties of LiMn1.5M0.5O4 (M=Al, Mg, Ni, Fe) and LiMn2O4/TiO2 nanocrystalline
electrode materials, J. Solid State Chem. 206 (2013) 257-264.
[20] Q. Luo, A. Manthiram, Effect of Low-Temperature Fluorine Doping on the Properties of Spinel LiMn2-2yLiyMyO4-ηFη (M = Fe, Co, and Zn) Cathodes, J.
Electrochem. Soc. 156 (2009) A84-A88.
[21] Y. Liu, T. Fujiwara, H. Yukawa, et al., Lithium intercalation and alloying efects on electronic structures of spinel lithium manganese oxides, Sol. Energ.
Mat. Sol. C. 62 (2000) 81-87.
[22] H.B. Sun, Y.G. Chen, C.H. Xu, et al., Electrochemical performance of rare-earth doped LiMn2O4 spinel cathode materials for Li-ion rechargeable battery, J.
Solid State Electrochem. 16 (2012) 1247-1254.
[23] A. Iturrondobeitia, A. Goñi, V. Palomares, et al., Effect of doping LiMn2O4 spinel with a tetravalent species such as Si(IV) versus with a trivalent species
such as Ga(III). Electrochemical, magnetic and ESR study, J. Power Sources 216 (2012) 482-488.
[24] S.C. Han, S.P. Singh, Y.H. Hwang, et al., Gadolinium-Doped LiMn2O4 Cathodes in Li Ion Batteries: Understanding the Stabilized Structure and Enhanced
Electrochemical Kinetics, J. Electrochem. Soc. 159 (2012) A1867-A1873.
[25] D.K. Lee, S.C. Han, D. Ahn, et al., Suppression of Phase Transition in LiTb0.01Mn1.99O4 Cathodes with Fast Li+ Diffusion, ACS Appl. Mater. Interfaces 4
(2012) 6842-6848.
[26] X. Liang, S. Zeng, Y. Liu, et al., Enhance cycling performance of LiMn2O4 cathode by Sr2+ and Cr3+ doping, Mater. Sci. Technol. 31 (2015) 443-447.
[27] S. Balaji, T. Mani Chandran, D. Mutharasu, A study on the influence of dysprosium cation substitution on the structural, morphological, and
electrochemical properties of lithium manganese oxide, Ionics 18 (2011) 549-558.
[28] S. Jayapal, R. Mariappan, S. Piraman, Dopant depends on morphological and electrochemical characteristics of LiMn2–XMoXO4 cathode nanoparticles, J.
Solid State Electrochem. 17 (2013) 2157-2165.
[29] B. Ebin, G. Lindbergh, S. Gürmen, Preparation and electrochemical properties of nanocrystalline LiBxMn2-xO4 cathode particles for Li-ion batteries by
ultrasonic spray pyrolysis method, J. Alloys Comp. 620 (2015) 399-406.
[30] S.H. Guo, S.C. Zhang, X.M. He, et al., Synthesis and Characterization of Sn-Doped LiMn2O4 Cathode Materials for Rechargeable Li-Ion Batteries, J.
Electrochem. Soc. 155 (2008) A760-A763.
[31] H.R. Lee, B. Lee, K. Y. Chung, et al., Scalable synthesis and electrochemical investigations of fluorine-doped lithium manganese spinel oxide, Electrochim.
Acta 136 (2014) 396-403.
[32] Y.K. Sun, B. Oh, H.J. Lee, Synthesis and electrochemical characterization of oxysulfide spinel LiAl0.15Mn1.85O3.97S0.03 cathode materials for rechargeable
batteries, Electrochim. Acta 46 (2000) 541-546.
[33] S.H. Ye, J.K. Bo, C.Z. Li, et al., Improvement of the high-rate discharge capability of phosphate-doped spinel LiMn2O4 by a hydrothermal method,
Electrochim. Acta 55 (2010) 2972-2977.
[34] Q.L. Jiang, K. Du, Y.B. Cao, et al., Synthesis and characterization of phosphate-modified LiMn2O4 cathode materials for Li-ion battery, Chinese Chem.
Lett. 21(2010) 1382-1386.
[35] S.M. Dou, Review and prospects of Mn-based spinel compounds as cathode materials for lithium-ion batteries, Ionics 21 (2015) 3001-3030.
[36] Q.L. Wei, X.Y. Wang, X.K. Yang, et al., The effects of crystal structure of the precursor MnO2 on electrochemical properties of spinel LiMn2O4, J. Solid
State Electrochem. 16 (2012) 3651-3659.
[37] L. Hernan, J. Morales, L. Sanchez, et al., Sol-gel derived Li–V–Mn–O spinels as cathodes for rechargeable lithium batteries, Solid State Ionics 133 (2000)
179-188.
[38] X.J. Yang, H. Kanoh, W.P. Tang, et al., Synthesis of Li1.33Mn1.67O4 spinels with different morphologies and their ion adsorptivities after delithiation, J.
Mater. Chem. 10 (2000) 1903-1909.
[39] W. Xu, A. Yuan, Y. Wang, Electrochemical studies of LiCrxFexMn2−2xO4 in an aqueous electrolyte, J. Solid State Electrochem. 16 (2011) 429-434.
[40] Y.K. Sun, G.S. Park, Y.S. Lee, et al., Structural Changes (Degradation) of Oxysulfide LiAl0.24Mn1.76O3.98S0.02 Spinel on High-Temperature Cycling, J.
Electrochem Soc. 148 (2001) A994-A998.
[41] P. Ragupathy, Understanding the role of manganese valence in 4V spinel cathodes for lithium-ion batteries: a systematic investigation , RSC Adv. 4 (2014)
670-675.
[42] D. Capsoni, M. Bini, G. Chiodelli, et al., Inhibition of Jahn–Teller cooperative distortion in spinel by LiMn2O4 transition metal ion doping, Phys. Chem.
Chem. Phys. 3 (2001) 2162-2166.
[43] R. Arabolla Rodríguez, Y. Mosqueda Laffita, E. Perez Cappe, et al., A new strategy toward enhancing the phosphate doping in LixMn2O4 cathode materials,
Ceram. Int. 40 (2014) 12413-12422.
[44] M. Prabu, M.V. Reddy, S. Selvasekarapandian, et al., (Li, Al)-co-doped spinel, Li(Li0.1Al0.1Mn1.8)O4 as high performance cathode for lithium ion batteries,
Electrochim. Acta 88 (2013) 745-755.
[45] J.M. Amarilla, K. Petrov, F. Picó, et al., Sucrose-aided combustion synthesis of nanosized LiMn1.99−y LiyM0.01O4 (M=Al3+, Ni2+, Cr3+, Co3+, y=0.01 and 0.06)
spinels Characterization and electrochemical behavior at 25 and at 55 ◦ C in rechargeable lithium cells, J. Power Sources 191 (2009) 591-600.
[46] W. Choi, A. Manthiram, Influence of Fluorine on the Electrochemical Performance of Spinel LiMn2−y−zLiyZnzO4−ηFη Cathodes, J. Electrochem Soc. 154
(2007) A614-A618.
[47] W. Choi, A. Manthiram, Comparison of Metal Ion Dissolutions from Lithium Ion Battery Cathodes, J. Electrochem Soc. 153(2006) A1760-A1764 .

SP
IP
20
40
60
80
100
2 / degree
Fig. 1. XRD patterns of the mixture SP and IP
5
0
20
b
a
10
0
DSC
0
0
DSC
-5
-10
-20
-30
TG
-10
-15
-20
-2
-4
TG
-40
-2
-50
-60
-70
0
200
400
600
800
1000
0
200
400
600
800
1000
Temperature/ ℃
Temperature/ ℃
2
8
c
1
0
-5
d
0
4
0
DSC
-2
-4
-6
-8
-10
-12
-1
-2
-3
-4
-5
0
-10
-15
-20
-25
DSC
TG
-4
-8
-12
TG
0
200
400
600
800
1000
0
200
400
600
800
1000
Temperature/ ℃
Temperature/ ℃
Fig. 2. TG/DSC of the raw materials LiOH·H2O (a), MnO2 precursor (b), mixtures SP (c) and IP (d)

Fig. 3. SEM images of MnO2 precursor (a) and as prepared LMO (b: SLM, c: LM, d: LLM, e: LLMN, f: LLMNP)
* Mn2O3
LLMNP
LLMN
LLM
LM
SLM
*
*
10
20
30
40
50
60
70
80
90 100
2 / degree
Fig. 4. XRD patterns of LMO samples
Mn 2p3/2
Mn 2p1/2
MnO2
LLMNP
LLMN
LLM
LM
SLM
Mn2O3
632 636 640 644 648 652 656 660 664 668
Binding energy / eV
Fig. 5. Mn 2p XPS spectra for LMO samples, in comparison with those for MnO2 and Mn2O3

4.4
4.2
4.0
3.8
3.6
3.4
3.2
3.0
4.4
4.2
4.0
3.8
3.6
3.4
3.2
3.0
a
b
0
20
40
60
80
100
120
140
0
20
40
60
80
100
120
140
Specific capacity / mAh g-1
Specific capacity / mAh g-1
4.4
4.4
4.2
4.0
3.8
3.6
3.4
3.2
3.0
d
c
4.2
4.0
3.8
3.6
3.4
3.2
3.0
0
20
40
60
80
100
120
140
0
20
40
60
80
100
120
140
Specific capacity / mAh g-1
Specific capacity / mAh g-1
4.4
4.2
4.0
3.8
3.6
3.4
3.2
3.0
e
0
20
40
60
80
100
120
140
Specific capacity / mAh g-1
Fig. 6. Charge and discharge curves of sample (a)SLM, (b)LM, (c)LLM, (d)LLMN, (e) LLMNP

130
120
110
100
90
130
120
110
100
90
a
b
LLMNP
LLMN
LLM
LM
SLM
LLMNP
LLMN
LLM
LM
SLM
80
80
70
70
0
10
20
30
40
50
0
10
20
30
40
50
Cycle number / n
Cycle number / n
Fig. 7. Cycling performance and coulombic efficiency of the prepared samples at different temperature (a: 25 °C, b: 55 °C)
140
0.2C
0.2C
1C
5C
120
100
80
60
40
20
0
10C
LLMNP
LLMN
LLM
LM
SLM
0
10
20
30
40
50
Cycle number / n
Fig. 8. Comparison of the rate capability among these samples