Low-Temperature Synthesis and Electrochemical Performance of Layered m-LiMnO2
Authors: Yu Denis Yau Wai , Zhou Hui , Li Yingshun , Zhang Jiaolong , Kang Wenpei 
Published: February 2020

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, LiNi1/3Co1/3Mn1/3O2 and LiNixCoyAlzO2 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 NaMnO2,
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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Figure 1