A Novel Li-Battery Cathode Material: Synthesis and Characterization of Li(Mn1-xCox)BO3

LiFePO₄ is very attractive as positive electrode material for lithium-ion batteries especially for its safety and low cost. However, its specific capacity is limited to 170 mAh/g. LiMBO₃ (M=Fe, Mn, Co) could be good alternatives to phosphates by having a higher theoretical specific capacity (>210 mAh/g) keeping the advantage of safety [1]. Moreover, borate compounds are expected to suffer from very low volume change upon cycling which is very interesting for the cycle life of the battery.
Nevertheless, the lithium intercalation potentials in borate compounds are lower than in phosphates which will limit their energy density [2]. Theoretically, LiCoBO₃ is the most interesting borate compound in term of operating voltage and specific capacity; however, approaching its theoretical capacity seems difficult due to the structural/chemical instability of fully delithiated LiCoBO₃ [3].

In order to have both reversible capacity and relatively high voltage, new Li(Mn_1-xCo_x)BO₃ have been explored. These materials have been synthesized for the first time by a multiple-step process. This one includes the synthesis of two intermediates essential to prevent the formation of metallic cobalt and Mn³⁺during the heat treatment.

Powders X-Ray diffraction patterns of Li(Mn_1-xCo_x)BO₃ and the evolution of lattice parameters with the cobalt content will be presented. TEM and neutron powder diffraction were carried out on these materials. Interestingly, ED patterns indicate that the C2/c space group previously reported for m-LiMnBO₃ [4] and LiCoBO₃ [5] cannot describe Li(Mn_1-xCo_x)BO₃ compounds. Neutron diffraction helps with the complete resolution of the structure and will be discussed during the presentation.

Electrochemical characterizations were also carried out on Li(Mn_1-xCo_x)BO₃ (0 ≤ x ≤ 1). Each material shows electrochemical activity, without in situ carbon coating. In order to improve performances of these compounds, an optimized synthesis has been developed and will be presented.

References:

[1] V. Legagneur et al., Solid State Ionics 139 (2001) 37

[2] D-H. Seo et al., Physical Review B 83, 205127 (2011)

[3] Y. Yamashita et al., ECS Electrochemistry Letters, 2 (8) A75-A77 (2013)

[4] O.S. Bondareva et al., Sov. Phys. Crystallogr. 23 (1978) 269

[5] Y. Piffard et al., Acta Cryst. (1998) C54, 1561-1563