A review of recent developments in the surface modification of LiMn2O4 as cathode material of power lithium-ion battery
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- Yi, T., Zhu, Y., Zhu, X. et al. Ionics (2009) 15: 779. doi:10.1007/s11581-009-0373-x
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LiMn2O4 (LMO) is a very attractive choice as cathode material for power lithium-ion batteries due to its economical and environmental advantages. However, LiMn2O4 in the 4-V region suffers from a poor cycling behavior. Recent research results confirm that modification by coating is an important method to achieve improved electrochemical performance of LMO, and the latest progress was reviewed in the paper. The surface treatment of LMO by coating oxides and nonoxide systems could decrease the surface area to retard the side reactions between the electrode and electrolyte and to further diminish the Mn dissolution during cycling test. At present, LiMn2O4 is the mainstreaming cathode material of power lithium-ion battery, and, especially the modified LMO, is the trend of development of power lithium-ion battery cathode material in the long term.
KeywordsPower lithium-ion batteryCathode materialLiMn2O4Surface modification
Lithium-ion batteries are regarded as promising new power sources for hybrid electric vehicles as well as for portable electronic devices due to their long cycle life and high energy density. Spinel LiMn2O4 is one of the most promising cathodes because of its low material cost, high natural abundance, environmental harmlessness, and good safety compared with the LiCoO2 used in current batteries [1–10]. Unfortunately, LiMn2O4 shows fast capacity fading during cycling, which has been an obstacle to its commercialization. The capacity fading has been ascribed to the following possible factors: (1) dissolution of Mn2+ , (2) Jahn–Teller distortion of Mn3+ ions , and (3) decomposition of electrolyte solution on the electrode . Of the above factors, dissolution of manganese into the electrolyte during cycling is believed to be the main one. To solve these problems, many researchers have studied the mechanism of capacity fading and have put forward some method to overcome capacity fading by doping the spinel with several cations, such as Al , Mg , transition metal ions [7–10], etc., to enhance the structural stability. However, Mn dissolution resulted from some side reactions that occurred at the interface between the electrode and the electrolyte during the charge/discharge process . As the cathode electrode contacted with the Li-based electrolyte directly in Li-ion batteries, Mn dissolution was induced by the generation of acids like hydrogen fluoride (HF), which was resulted from the reactions of fluorinated anions with the manufacture of instable Li-based salt  and solvent oxidation . In order to solve this problem, surface modification of the cathode electrode is an effective way to reduce the side reactions. By coating oxides and nonoxide systems, the surface treatment of spinel LiMn2O4 could decrease the surface area to retard the side reactions between the electrode and electrolyte and to further diminish the Mn dissolution during cycling test. Here, we reviewed the recent progress on this aspect, and future directions were pointed out.
LMO coated by oxides systems
Synthesis methods and the electrochemical performance of LMO coated by metal oxide
Coated oxides and references
LMO powder was prepared by the sol–gel method calcinated at 850 °C for 15 h. SiO2-coated LMO were synthesized by polymeric process at the calculation of 1.0, 2.0, and 3.0 wt.% by using silicic acid as the coating of raw materials.
2.0 wt.% of SiO2-coated LMO has significantly improved the capacity retention and excellent cycleability for 30 °C and 60 °C compared with the uncoated LMO because the formation of a passive layer film during electrochemical cycling is controlled.
LMO powder was obtained from Merck KGaA (highly pure, Li battery grade). MgO-coated LMO was synthesized as follows: the mixed solution of LMO and (CH3COO)2 Mg∙4H2O is added with aqueous ammonia, then centrifuged, and washed. The formed product was further dried at 450 °C under air for 4 h.
The capacity fading of MgO-coated LMO electrodes at elevated temperatures is much smaller compared to regular LMO electrodes.
LMO was synthesized by a solid-state method. LMO was added into the mixed solution of zinc acetate and triethanolamine and evaporated at 80 °C until a black gel formed. The gel was dried at 100 °C for 2 h and calcined for 6 h at 500 °C to obtain the ZnO-coated LMO.
Uncoated LMO delivered an average capacity loss of 0.81% per cycle in 50 cycles; the 1 wt.%, 2 wt.%, and 5 wt.% ZnO-coated LMO only showed the average capacity loss of 0.54%, 0.19%, and 0.14% per cycle, respectively, under a current rate of C/2 at 55 °C between 3.4 and 4.3 V.
LMO was synthesized by a solid-state method. For preparing CeO2-coated LMO powder by using a polymeric precursor based on the Pechini method.
The initial discharge capacity was decreased with the increased amount of CeO2 coating, and 2% CeO2-coated LMO exhibits a slight decrease in its original specific capacity of 107 mAh g−1 and excellent capacity retention (more than 82% of its initial capacity).
LMO was from Sedema(Belgium). For making the Zr oxide coating layer, Zr butoxide was mixed with 1-butanol in a volume ratio of alkoxide/alcohol = 1:4 under ultrasonic agitation for 30 min. LMO powder was then dispersed into the coating solution, followed by settling under vacuum. The dispersion solution was then evaporated. Finally, the coated powder was calcined at 400 °C, ground, and sieved.
5 wt % ZrO2-coated LMO shows tremendous enhancement in cycling stability at CD rates up to 10 C, and the coated spinel electrode exhibits a lower cubic-tetragonal transition potential, a smaller charge-transfer impedance by 4- to 5-fold, and it profoundly reduces lattice contraction by 66% upon charge.
A sol precursor for coating was prepared by mixing ethyl alcohol and AlCl3·6H2O (Junsei, Japan) and then LiMn2−xMxO4 (Nikki, Japan, M = Zr, reversible capacity: 100 mAh g−1) was immersed in sol precursor. After drying at 80 °C, the powder obtained was calcinated at 500 °C for 3 h.
The initial capacity of Al2O3-coated LMO is higher than that of the bare LMO, and the cell performance was enhanced with the Al2O3 coating. Al2O3-coated samples have improved interfacial properties between the electrolyte and electrode.
Co–Al mixed metal oxide 
LMO powder was provided by Shijiazhuang Best BatteryMaterial Co., Ltd. (China). The mixed solution of LMO, Co(NO3)2·6H2O and Al(NO3)3·9H2O is added with LiOH·H2O solution to maintain the mixture at pH 10.5 with vigorous agitation for 3 h, and then filtered and dried at 120 °C for 12 h, and subsequently heat-treated in a furnace at different temperatures for 5 h in air. The amount of Co in the coating solution was varied from 2 to 4 wt.% (based on LMO) while the amount of Al in the coating solution was fixed at 0.5 wt.% (based on LMO).
The CoAl-MMO-coated (3 wt.% Co and 0.5 wt.% Al based on LMO) LMO after heat treatment at 400 °C shows the best cycling stability with a specific discharge capacity of 100 mAh g−1 and 92.2 mAhg−1 after 50 cycles at 25 °C and 55 °C, respectively. This is much higher than that of the pristine LMO (97.4 and 73.7 mAhg−1 at 25 °C and 55 °C, respectively).
It can be concluded that the surface coating of LMO with metal oxides could be an effective way to improve its electrochemical performance at elevated temperatures in practical batteries.
LMO coated by nonoxide systems
The nonoxide systems include metal phosphates, metal, other electrode materials, carbon, fluoride, or other novel materials.
It has been reported that the AlPO4 coating exhibited better thermal stability than the metal oxide coating for lithium-ion battery [26, 27]. Hence, it can be expected that AlPO4-coated LMO has an improved electrochemical performance. AlPO4-coated LMO was synthesized as follows : LMO powders (prepared by solid-state method) were slowly dispersed into the AlPO4 solution under constant stirring for 5 h, and then the mixed slurry was dried in an oven for 2 h at 100 °C and subsequently annealed at 700 °C under air for 3 h in a furnace. The uncoated LiMO showed 17.9% and 32.9% capacity loss in 50 cycles at 30 °C and 55 °C, respectively; the AlPO4-coated LMO only exhibited the capacity loss of 2.6% and 7.6% at 30 °C and 55 °C, respectively. The improvement of cycleability is ascribed to the AlPO4 film, minimizing the contact area of LiMn2O4/electrolyte interface, thus, suppressing the dissolution of Mn effectively .
It is well known that gold and silver belong to the lowest-resistance metals; hence, they can be expected to enhance electron conduction of coated LMO and then improve its electrochemical performance. Tu et al.  have reported that a nano-gold film-coated LMO by ion sputtering method shows better capacity retention at room temperature than that of uncoated LMO, which is attributed to reduce contact area of electrode/electrolyte interface and suppressed dissolution of manganese during electrochemical cycling. Zhou et al.  reported that the initial discharge capacity was decreased with increasing the amount of Ag coating, but Ag (0.1)/LMO exhibits the highest discharge capacity after 40 cycles 108 mAh g−1 among all samples. Son et al.  also reported that the silver-coated nanoparticle LMO (3.2 wt.% Ag) shows excellent cycleability at 2 C galvanostatic conditions. It can be concluded that the improved cycleability of metal coating LMO can be attributed to enhanced electron conduction between LMO particles because of the low resistance of silver and gold.
Synthesis methods and the electrochemical performance coated by other electrode materials
Other electrode coating materials [references]
Stoichiometric amounts of lithium acetate and cobalt acetate were dissolved in distilled water. An aqueous glycolic acid water solution was then added to this mixture solution to produce a gel-type solution, and the pH value was controlled at 6.5–7.0. The resultant solution was evaporated until its concentration reached about 1 mol/l. The commercial LMO powder was then added to this resultant coating solution while stirring, and then screened with a centrifuge. The screened powder was dried in a vacuum oven and calcined for 6 h at 800 °C.
LiCoO2-coated LMO showed a higher discharge capacity of 120 mAh/g than LiMn2O4 (115 mAh/g). LMO maintained only 50% of its maximum capacity at a 20-C rate (2400 mA/g); the LiCoO2-coated LMO maintained more than 80% of maximum capacity. LiCoO2-coated LMO with 3 wt.% conducting agent (acetylene black) showed the higher rate capability than as-received LMO with 20 wt.% conducting agent.
Stoichiometric amounts of acetate and glycolic acid were mixed in distilled water according to priority, and the pH was controlled 6.5–7.0. The resultant solution concentration of was controlled at 0.7–1 mol/l. Commercial LMO was then added to this coating solution with a constant stirring. The powder in the coating solution was screened with a centrifuge to remove the remaining coating solution. The screened powder was dried in a vacuum oven and was calcined for 6 h at 750 °C in oxygen atmosphere.
The coated LMO showed an excellent capacity retention at 65 °C compared with pure LMO. The capacity of pure LMO decreased drastically with cycling at 65 °C, and LiNi0.8Co0.2O2-coated LMO shows lower 0.08% per cycle loss. LiNi0.8Co0.2O2-coating is a very effective in improving the elevated temperature properties.
LMO was prepared by a citric acid-assisted sol–gel method. Tetrabutyltitanate, lithium acetate, and acetic acid were dissolved in a mixed solution containing ethanol and distilled water according to priority. And then, the as-prepared LMO was added to the previously mentioned sol under stirring. The gelatin so formed was dried at 100 °C for 1 h and fired at 800 °C for 1 h to obtain the final powders.
The LMO delivered a discharge capacity of 116 mAh g−1 at the first cycle and remained only 71.4 mAh g−1 after 45 cycles. The capacity loss was about 0.99% at 55 °C. However, 0.62% and 0.45% capacity loss per cycle were found for 2 and 5 mol% LTO-coated LMO. The improvement of electrochemical performance is attributed to the suppression of electrolyte decomposition on the surface of LMO.
By a tartaric acid gel method
In comparison with the unmodified LMO, the LiNi0.05Mn1.95O4-modified LMO exhibited excellent electrochemical characteristics, the same initial discharge capacity of 125 mAh g−1, high charging–discharging efficiency, and good cycle stability.
The precursor of LMO was calcined at 600 °C for 10 h and mixed with Cu(CH3COO)2 in deionized water. The mixture powders were then calcined at 870 °C for 10 h to synthesize LiCuxMn2−xO4-coated LMO composite.
LiCuxMn2−xO4-coated LMO composite cathode material exhibited better electrochemical performance than the base LMO, especially at high C rates.
Carbon coating has been known to be effective not only in enhancing the electrical conductivity of metal oxides but also in increasing their absorbing ability against organic molecules. In addition, a coated carbon layer would protect the metal oxides from chemical corrosion. Han et al. reported that  the coated carbon layer composed of disordered amorphous carbon and polycyclic aromatic hydrocarbons can modify the cubic spinel-type atomic arrangement of lithium manganate, and that the carbon coating can improve the electrode performance of spinel lithium manganate because of the increase of grain connectivity and/or the protection of manganese oxide from chemical corrosion. Patey et al.  reported that LMO/carbon nanocomposites had a considerably higher specific galvanostatic discharge capacity at a 5-C rate or greater than the electrode with powder of pure LMO, and the specific energy of a thin-layer lithium-ion battery containing the flame-made LMO/carbon nanocomposite as positive electrode and LiC6 as negative electrode (78 Wh kg−1 at 50-C rate).
Fluoride is also used to coat LMO to improve its cycleability because it is very stable even in HF. Li et al.  reported that the discharge capacity of LMO decreases slightly with increasing the amount of the coated SrF2 to 2.0%, but the cycleability of LMO at elevated temperature is improved obviously. LMO remains only 79% of its initial capacity after 20 cycles, whereas the 2.0% (molar fraction) coated LMO shows 97% of its initial capacity retention cycle at 55 °C. Lee et al.  reported that the BiOF-coated spinel Li1.1Al0.05Mn1.85O4 electrode had excellent capacity retention at 55 °C, maintaining its initial discharge capacity of 96.1% after 100 cycles while that of the pristine material was only 84.4% compared with the initial discharge capacity.
The polymer possesses the antioxidative capability, and slowly expands instead of dissolving while dipping it in the electrolyte for a long time. As a result, the modified LMO-based cathode displays an improved stability during repeated charge/discharge in organic electrolyte at an elevated temperature [50, 51]. Hu et al.  reported that the electrochemical storage properties of the spinel at 55 °C based on the LMO film surface decorated with the functional polymer was improved, and the 45th discharge capacity was improved at 55 °C from 56.8 to 81.4 mAh g−1 on the LMO electrode. Arbizzani et al.  reported that poly(3,4-ethylenedioxy) thiophene (pEDOT) can function as an electronic conductor and substitute the carbon usually mixed with the inorganic oxide-based electrodes to improve the electronic conductivity of nonstoichiometric Li1.03Mn1.97O4 spinel, and the reversible capacity and capacity retention are increased.
From the above illustrations, it can be concluded that coated LMO is one of the promising cathode materials for power lithium-ion batteries for electric vehicles since they show excellent performances, such as high capacity, good cycleability, high rate capability, high thermal stability, and high-temperature performance. Surface coatings such as metal oxide and other compounds/composites on LMO can prevent the direct contact of electrolyte solutions with cathode materials, reduce the generation of acids like HF, improve structural stability, and suppress phase transitions. It is sure that the surface coating of LMO cathode materials will play, more and more, an important role in improving its electrochemical performance. Better and/or cheaper LMO cathode materials from surface modification will come up in the near future [52–55]. At present, LMO is the mainstreaming cathode material of power lithium-ion battery, and, especially the modified LMO, is the trend of development of power lithium-ion battery cathode material in the long term.