Study of Inexpensive Li2Fe1-XMnxSiO4 Cathode Materials Synthesised Using Abundant Materials By Conventional Methods

Introduction

Electrochemical storage units such as batteries and capacitors are used to store electrical energy for a wide range of applications. Different battery chemistries can be selected depending on their applications. The last decades, Li ion batteries have been widely introduced in e.g. hybrid and electrical vehicles.

In the Li ion battery, the active electrode materials typically consist of metal oxide as the cathode and graphite as the anode where both have the ability to intercalate Li ions. For a more widespread use of Li ion batteries, either price has to decrease or the performance must be enhanced. Capacity can be increased by different means, such as replacing graphite with silicon by different methods and/or by changing the cathode materials. However, more effort should be focused on the cathode electrode chemistry, since this electrode has higher cost, use less environmentally benign materials and have a lower capacity compared to the anode.

The introduction of LiFePO₄has partly solved this problem, by removing the expensive, toxic and less abundant metals. However, this cathode material has challenges related to poor electronic conductivity and introduction of carbon and the importance of nanosized particles make the process more complex and expensive.

The orthosilicates Li₂(Fe, Mn, Co)SiO₄ have been proposed as a promising family of cathode materials with possible additional benefits compared to LiFePO₄ such as two available Li ions per formula unit for charge transport and better stability due to the SiO₄ group. However, the orthosilicates with the even lower conductivity compared to LiFePO₄and challenges related to the existence of several stable crystallographic polymorphs must be solved. Preferably this should be accomplished using inexpensive processing routes and precursors. Here, we present possible ways to accomplish this by using raw materials readily available in Norway, combined with conventional processing routes. The cathode materials are compared with similar materials synthesized by other complex routes.

Experimental

Li₂Fe_1-xMn_xSiO₄/C composites where prepared by mixing Li₂CO₃, iron and/or manganese source, silica source and carbon source. The different sources were

• Fe₃O₄ submicron powders and Fe-oxalate
• Mn-oxide based dust / Mn-oxide chemical grade.
• Fused silica and silica waste from Si production
• Sucrose and glucose.

The pressed pellets were heat treated in an argon atmosphere at optimized temperature and time to form as phase pure active cathode material as possible.

The composites were examined for phase purity (XRD) and morphology (SEM and TEM). The electrochemical performance Li₂Fe_1-xMn_xSiO₄ cathode materials was studied using a CR2016 coin cell consisting of a Li foil anode, Celgard 2320 separator and 1.0M LiPF₆in 3:7 EC:DEC electrolyte. Charge/discharge behavior was studied by galvanostatic cycling to evaluate the charge capacity and stability while the insertion and extraction reactions were studied by cyclic voltammetry. The electrochemical characteristics of the different batteries are discussed and related to the synthesis techniques and different precursors of the cathodes.

Acknowledgement

Financial support is gratefully acknowledged from the Research Council of Norway RENERGI program, grant number 216469/E20.