517 ISSN 1023-1935, Russian Journal of Electrochemistry, 2019, Vol. 55, No. 6, pp. 517–523. © Pleiades Publishing, Ltd., 2019. Published in Russian in Elektrokhimiya, 2019, Vol. 55, No. 6, pp. 687–695. Microstructural Influence on Electrochemical Properties of LiFePO 4 /C/Reduced Graphene Oxide Composite Cathode 1, 2 G. Kucinskis a, *, G. Bajars a, **, K. Bikova a , K. Kaprans a , and J. Kleperis a a Institute of Solid State Physics, University of Latvia, Riga, LV-1063 Latvia *e-mail: gints.kucinskis@cfi.lu.lv **e-mail: gunars.bajars@gmail.com Received October 11, 2018; revised November 30, 2018; accepted January 22, 2019 Abstract—LiFePO 4 /C/reduced graphene oxide (rGO) composites with different morphologies were synthesized, allowing evaluation of the electrochemical performance as a function of the sample morphology. LiFePO 4 parti- cles anchored on rGO sheets and rGO sheets wrapping LiFePO 4 agglomerations were two of the most pro- nounced features observed. The structure with LiFePO 4 particles anchored on rGO sheets was found to be the most optimal and give rise to both increased capacity and improved rate capability. Keywords: LiFePO 4 , graphene, lithium ion batteries, cathode, morphology DOI: 10.1134/S1023193519060120 INTRODUCTION There are limited number of lithium insertion cath- ode materials with capacity and stability able to meet the current requirements for lithium ion batteries. The commonly used material classes are layered transition metal oxides (including LiCoO 2 [1], NCM or LiNi 0.33 Co 0.33 Mn 0.33 O 2 [2, 3], Li 2 MnO 3 -stabilized cathodes [4] and further variations of the same struc- ture), LiMn 2 O 4 spinel-type [5, 6] and LiFePO 4 [7]. Among these, LiFePO 4 (LFP) possesses theoretical capacity 170 mA h g –1 , has high chemical and cycling stability, low raw materials’ cost and is comparably environmentally benign [8]. Although the gravimetric charge capacity is a fixed value, improvements in rate capability and cycling sta- bility of electrodes are still possible. They are mostly achieved by using electron conducting additives [9‒11] and optimizing electrode architecture, includ- ing mixing [12], porosity [13] and thickness [14]. Elec- tron conducting additives are often carbon-based materials due to their high electronic conductivity, stability and low cost [15]. Carbon coatings [16, 17], carbon fibers [18], nanotubes [19, 20] and various other porous structures have been widely studied. The electron conducting additives create a network within the electrode, ensuring that electron conduction path- ways are available between the current collector and vir- tually every grain in the electrode. This leads to both increased rate capability and improved cycling stability. Since the discovery of graphene and its superior electron conduction properties [21], there have also been attempts to use it in lithium ion batteries – for a thorough literature review the reader can be referred to reference [22]. Reduced graphene oxide (rGO), a close relative of graphene, is a monolayer of graphite, consisting of sp 2 hybridized carbon atoms arranged in a honeycomb crystal lattice [22]. rGO contains more lattice defects and often also additional functional groups, since it is obtained by reducing graphene oxide (GO), a material with graphene-like carbon honey- comb lattice decorated by hydroxyl and epoxy func- tional groups as well as some carbonyl groups (car- boxyl, carbonyl, ester) along the GO sheet edge [23, 24]. Often rGO sheets used consist of several lay- ers. However, since GO is more hydrophilic than rGO, there is the benefit of GO mixing more readily with the nanoparticles of the cathode material [25]. Although there have been many studies analyzing graphene as an electron conducting additive for lith- ium ion batteries [25–52], there is a lack of agreement on what the optimal preparation process or mixing of both compounds is. Often different studies use differ- ent LiFePO 4 preparation techniques, yielding differ- ing grain sizes, varying amounts of carbon coating and other electrode additives. Moreover, it is challenging to evaluate rGO used in each of these studies, as dif- ferent defect concentrations, sheet sizes and thick- nesses could lead to varying electron conduction val- ues of rGO [53]. This makes the comparison between various studies difficult as far as evaluating the mor- phology effects and electrochemical properties go. In our work we mix GO with LiFePO 4 or its precursor at various synthesis steps while maintaining the same 1 The article is published in the original. 2 Based on the paper presented at the XIV Meeting “Fundamental Problems of Solid State Ionics,” Chernogolovka (Russia), Sep- tember 9–13, 2018.