Tuning MnOx Phase in GO@MnOx Composites Using Heat-Treatments to Improve Lithium Ion Battery Performances
Authors: Jung Wonsang , Lee Seungmin , Joh Han-Ik , Eom KwangSup , Fuller Thomas F 
Published: February 2020

Electric vehicles (EVs) and energy storage systems (ESSs) require electrode materials with low cost and high mass-production efficiency via easy synthesis methods as well as high capacity and stability. In the respect, manganese oxide (MnOx) is an attractive material because it is abundant, non-toxic, and has low cost. MnOx as an electrode material for the lithium ion battery (LiB) has attracted much attention. However, during LiB cycling, MnOx is easily pulverized and detached from electrode, which causes severe capacity fade of a cell.

In this study, graphene oxide (GO) and manganese oxide composite (MnOx) with cubic shape; that is, GO@MnOx, is prepared with a facile, one-pot chemical synthesis method. Fig. 1a shows HR-TEM and STEM mapping images of the as-prepared GO@MnOx. In particular, not only does GO increase the electrical conductivity, but it also provides a template where MnOx is anchored, which alleviates pulverization of MnOx during de/lithiation.

The GO@MnOx is heat-treated at 0, 200, 400, and 600℃, respectively. As shown in Fig. 1b, the crystalline structure of GO@MnOx heat-treated at 0 and 200℃ is Mn3O4; that is, hausmannite, and that heat-treated at 400℃ is composed of Mn3O4 and MnO simultaneously, and that heat-treated at 600 ℃ is MnO, that is, manganosite. According to the crystalline structure, specific capacity of GO@MnOx is different. In particular, the GO@MnO heat-treated at 600oC shows the highest specific capacity of 850 mAh g-1 and the most stable cycling performance. Contrary to the GO@MnO, the GO@Mn3O4 shows 480 mAh g-1 (Fig. 2).

In addition, it is notable that the specific capacities of all kinds of GO@MnOx increase with initial cycling between 50 and 150 cycles. The behavior is probably due to pulverization by volume expansion (~ 200%) of active materials (but securely anchored on GO) and changes in the structure from crystalline to disordered amorphous, which provides more facile lithium diffusion paths. For Mn3O4, when it is firstly lithiated, it changes to LiMn3O4, which has a relatively low ionic conductivity, and thus, decreases lithium diffusion rate. However, through further repeated lithiation/delithiation, Mn3O4 transforms to MnO and Li2O, as shown in Fig.3. In contrast, for MnO, it needs to take one step lithiation to form Mn cluster and Li2O, namely, to be fully lithiated. Therefore, GO@MnOx containing MnO shows a fast capacity increase within a few cycles.

Considering the convoluted nature of MnOx related to capacity increases and fading, it is speculated that for Mn3O4 initial one more lithium insertion contributes to larger theoretical capacity but, paradoxically, more lithiation induces more volume expansion leading to loss of active materials and hence, less capacity increase. Moreover, inserted Li2O in MnO crystals produced at initial lithiation presumably has negative impact on connection of octahedrons of MnO. On the other hand, for MnO, during de/lithiation MnO octahedrons are probably well connected, thus it is possible to have relatively high capacity due to less active materials loss.

Figure 1