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 (MnO_x) is an attractive material because it is abundant, non-toxic, and has low cost. MnO_x as an electrode material for the lithium ion battery (LiB) has attracted much attention. However, during LiB cycling, MnO_x 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 (MnO_x) with cubic shape; that is, GO@MnO_x, is prepared with a facile, one-pot chemical synthesis method. Fig. 1a shows HR-TEM and STEM mapping images of the as-prepared GO@MnO_x. In particular, not only does GO increase the electrical conductivity, but it also provides a template where MnO_x is anchored, which alleviates pulverization of MnO_x during de/lithiation.

The GO@MnO_x is heat-treated at 0, 200, 400, and 600℃, respectively. As shown in Fig. 1b, the crystalline structure of GO@MnO_x 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@MnO_x is different. In particular, the GO@MnO heat-treated at 600^oC 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@MnO_x 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@MnO_x containing MnO shows a fast capacity increase within a few cycles.

Considering the convoluted nature of MnO_x 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.