Low-Temperature Synthesis and Electrochemical Performance of Layered m-LiMnO2

Layered lithium transition metal compounds (LiMO₂) are attractive as cathode material for lithium-ion batteries because the structure allows effective insertion and removal of lithium during charge and discharge.¹ Among the different transition metals, LiCoO₂, LiNi_1/3Co_1/3Mn_1/3O₂ and LiNi_xCo_yAl_zO₂ materials are already commercialized. To reduce the toxicity and the cost of the materials, LiMnO₂ was extensively studied in the 1990s without much success.^2,3 This is mainly because layered LiMnO₂ cannot be easily made. Spinel LiMn₂O₄ and orthorhombic LiMnO₂ are the typical phases that are formed when the material is synthesized in air or inert atmosphere.⁴ Metastable monoclinic LiMnO₂ (m-LiMnO₂) was previously synthesized by ion-exchange from NaMnO_2,^{2, 5} or high-temperature synthesis with Al doping.⁶ These methods however does not allow much control of the resulting m-LiMnO₂material.

Here we report for the first time a direct synthesis of layered m-LiMnO₂ through a carbo-thermal reduction method with LiOH and MnO₂ as precursors under Ar atmosphere at 450°C. As shown in Figure 1(a), the XRD patterns of our synthesized LiMnO₂ sample coincide well with the standard of monoclinic LiMnO₂ (ICDD PDF#87-1255), which has a layered structure similar to LiCoO₂ but with a monoclinic distortion induced by the Jahn-Teller effect on high spin Mn³⁺.^{2, 5} The sample was tested with Li metal as counter electrode between 2 and 4.5V vs. Li/Li⁺ and the results are shown in Figure 1(b) and (c). Initial capacity is close to 200 mAh/g. Even after 40 cycles, the capacity of above 150 mAh/g can be maintained (Fig. 1(c)). With cycling, plateaus at 3V and 4V are developed, suggesting migration of Mn within the structure with cycling.

In comparison, LiMn₂O₄ and o-LiMnO₂ were tested within the same voltage range. Capacity of LiMn₂O₄ dropped with cycling, which is attributed to the formation of the tetragonal phase below 3V, Jahn-Teller distortion and Mn dissolution. On the other hand, o-LiMnO₂ initially shows a small capacity because Li path is blocked by the orthorhombic structure. An increase in capacity is observed with cycling due to increase utilization of the LiMnO₂ structure. However, even after 40 cycles, the capacity was just increased to 100 mAh/g, indicating the o-LiMnO₂was not fully utilized. 

Among the three samples, m-LiMnO₂ shows the best cycle stability, suggesting less Mn migration for the layered material. Transformation to spinel-like structure during the cycling is still an issue for m-LiMnO₂. Doping and surface coating are being carried out to suppress the phase transformation and the details will be discussed at the meeting.

Our low-temperature method provides an easy way to synthesize the material with the desirable structure. This allows us to manipulate its composition and morphology to improve the material further.  

Acknowledgement

This research is sponsored by City University of Hong Kong (Grant #7200370), Hong Kong SAR.

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