Remarkable efforts have been put forth for the development of environmentally benign, abundant, low-cost and high-performance cathode materials for Li-ion batteries. Spinel LiMn₂O₄ has been considered as the most promising cathode among the many candidates for next-generation energy storage systems due to afore-mentioned advantages[1]. However, there are some challenges associated to this cathode that inhibit its practical usage for future applications. During cycling, the structural change from cubic to tetragonal takes place, in the Mn³⁺ configuration due to Jahn-teller distortion followed by Mn²⁺ dissolution under the influence of acidic electrolyte which destabilises the cathode structure[2]. These changes lead to cathode’s structural transformation and particle cracking resulting in active material loss and capacity fading. The deposition of soluble Mn²⁺ on anode's SEI results in increased cell’s impedance[3]. Moreover, severe capacity fading at elevated temperatures is still an issue hindering its full-scale commercial development due surface manganese dissolution in the electrolyte at those temperatures[4]. Ever since the emergence of LiMn₂O₄, various strategies have been employed to exploit its potential. Among many techniques, surface-coating, material composites and doping have been widely used[5]. Most of these techniques focus either on the restriction of direct contact between the active material and electrolyte via coating or the structural enhancement to accommodate for the anisotropic volume changes caused due to disproportionation reaction. Although, these strategies have shown some positive impact on the cycle life of the LMO based Li-ion battery but, in most of the cases the protection from the acidic electrolyte and structural strengthening of LMO has not been addressed simultaneously. Therefore, In order to develop a practical LMO based Li-ion battery, the active material requires a robust protection from the acidic electrolyte contact via a continuous, uniform and Li-ion permeable coating with minimum mismatch while enhancing the structural stability simultaneously to mitigate the irreversible phase transitions.

Herein, we report a novel heterostructure design; NMC layered Li-ion permeable phase grown on the surface of Lithium-rich LiMn₂O₄ octahedra. The layered surface phase protects the host spinel from being directly exposed to the acidic electrolyte during electrochemical cycling. In addition, it provides an efficient path for the ionic and electronic mobility resulting in improved kinetics due to its Li-ion permeability. On the other hand, the excess Li in LMO contributes to the structural enhancement during cycling to accommodate anisotropic volume changes, thus resulting in a robust cathode for high-voltage Li-ion batteries. In comparison to spinel LMO, the newly modified LMO displays an enhanced cyclic performance with superior charge-discharge rate capability. The uniquely developed LiMn₂O₄ phase surface coated with layered structure demonstrated discharge capacity of 120 mAh g^-1 at 20 ℃ temperature while retaining >97% of its initial capacity after 300 cycles at 0.5C. Further, The cathode was tested at elevated temperatures of 60 °C, showing stabilised reversible specific capacity of 113 m Ah g^-1 at 0.2 C-rate ensuring energy density of 452 W h kg^-1. The full-cell utilising MCMB anode and newly modified LMO cathode showed the areal capacity of 1.22 m Ah cm^-2, after 100 cycles with a capacity retention of 95.3% at 0.44 mA cm^-2 while maintaining its trend till 300 cycles.

References:

[1] J. Gummow, A. de Kock, M. M. Thackeray, Solid State Ionics 69 (1994) 59.

[2] C. Hunter, J. Solid State Chem. 39 (1981) 142.

[3] Blyr, A. D. Pasquier, G. Amatucci, J. M. Tarascon, Ionics 09 (1997) 321.

[4] M. Thackeray, J. Cho, J. Electrochem. Soc. 146 (1999) 3577.

[5] Xiao, D. Ahn, Z. Liu, J-H. Kim, P. Lu. Electrochem. Commun. 32 (2013) 31-34.