Introduction

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], (2) Jahn–Teller distortion of Mn3+ ions [3], and (3) decomposition of electrolyte solution on the electrode [4]. 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 [5], Mg [6], 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 [11]. 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 [12] and solvent oxidation [13]. 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

Oxides coating over LiMn2O4 (LMO) can suppress the dissolution reaction of manganese ions at elevated temperature and clearly improve the cycleability of the spinel LiMn2O4 cathode materials. The coated oxides include nano-SiO2 [14], MgO [15, 16], ZnO [17–20], CeO2 [21], ZrO2 [22, 23], Al2O3 [23, 24], and Co–Al mixed metal oxide [25]. The schematic diagram of the metal oxide coating LMO procedure can be shown in Fig. 1 [15]. The synthesis methods and the electrochemical performance of LMO coated by oxides are shown in Table 1.

Fig.1
figure 1

Schematic diagram of the metal oxide coating LMO procedure from [15]

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Table 1 Synthesis methods and the electrochemical performance of LMO coated by metal oxide
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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.

Metal phosphates

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 [28]: 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 [28].

Metal

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. [29] 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. [30] 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. [31] 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.

Electrode materials

The LMO surface coated with other electrode materials can probably be an effective way to improve the electrochemical performance at room temperature and elevated temperature. The reason for the improved elevated temperature properties of LMO coated by other electrode materials is that the surface coating reduces the dissolution of Mn, which results from the suppression of the electrolyte decomposition. The coated electrode materials include LiCoO2 performed by sol–gel methods [32–34] and microemulsion method [35], LiNi0.8Co0.2O2 [36], Li4Ti5O12 [37, 38], LiNi0.05Mn1.95O4 [39], and LiCu x Mn2−x O4 [40]. The synthesis methods and the electrochemical performance of coating by other electrode materials are plotted in Table 2.

Table 2 Synthesis methods and the electrochemical performance coated by other electrode materials
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Carbon materials

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 [41] 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. [42] 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

Fluoride is also used to coat LMO to improve its cycleability because it is very stable even in HF. Li et al. [43] 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. [44] 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.

Novel materials

It is well known that molten Li2O–2B2O3 (LBO) compositions exhibit a combination of good wetting properties and relatively low viscosity in the molten state and also exhibit good ionic conductivity [45, 46]; LBO materials also are stable against the high oxidation potentials of the 4-V positive electrode materials used in Li-ion batteries. The side reaction and Mn dissolution between the interface of the cathode electrode and electrolyte was reduced significantly by surface modification of LBO glass in the LMO. Chan et al. [47] have reported that LMO cathode materials coated with LBO via solid-state method exhibited relatively good cycling performance, but the capacity fade was still 2.63% after 10 cycles at a current rate of 0.1 C. Şahan et al. [48] reported that the capacity retention of LBO-coated LMO via solid-state method is 7.5% after 30 cycles, and LBO-coated LMO electrode via solution method has an excellent cycling behavior without any capacity loss even after 30 cycles at room temperature and a 1-C rate as plotted in Fig. 2. Chan et al. [49] have also reported that Li1.08Mn2O4 cathode materials coated with LBO have a better high-temperature performance than that of Li1.08Mn2O4. The LBO-coated cathode powder with the fading rate of only 7% after 25 cycles showed better cycleability than the base one with the fading rate of 17% after 25 cycles at higher temperature.

Fig. 2
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Cycleability of all LMO materials at1-C discharge rate at room temperature from [47]

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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. [50] 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. [51] 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.

Prospect

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.