In search of high energy density negative electrode materials for sodium ion batteries

Thesis (Ph. D.)--University of Rochester. Department of Chemical Engineering, 2015.Lithium ion battery technology has become more and more mature these days, and we have started to see lithium ion batteries being widely used in pocket electronics and even in electric vehicles. However, lithium reserves are limited, and one may expect a lithium supply shortage if electric vehicles powered by lithium ion batteries become more and more popular. On the other hand, sodium is an ideal replacement for lithium since it shares many physical and chemical properties with lithium, and is also abundant. As a result, sodium ion batteries currently attract huge scientific and technological interest. However, while lots of cathode materials had been investigated for sodium ion batteries (SIBs), there has been relatively little exploration on anode materials for SIBs. Anode materials can be separated into three major types based on their electrochemical reaction mechanisms, which are intercalation, alloying and conversion reactions, respectively. In this thesis, all of these three types of anodes were investigated. Detailed electrochemistry will be discussed in following chapters. Intercalation materials studied consist of carbon based materials (spherical carbon and spherical hard carbon) and titanium based layered materials (sodium titanate nanotubes). Spherical carbon synthesized by the autogenic approach has good rate capability. Further investigation by NMR reveals that most of the capacity is coming from sodium intercalation into the graphene layers with enlarged layer spacing, which explains the excellent rate performance. However, the capacity is relatively low compared to hard carbon, only 100 mAh g-1 being achieved for this type of material. In order to obtain high capacity, microporous spherical hard carbon was synthesized by PFA pyrolysis. The reversible capacity is significantly improved to 253 mAh g-1 at 20 mA g-1 rate. 100 mAh g-1 capacity can be well sustained for 1000 cycles without any fade at 1C, indicating its excellent long term cyclability. Na2-xHxTi2O4(OH)2 nanotubes (orthorhombic unit cell, Immm space group) were synthesized by hydro-thermal reaction using NaOH and anatase as starting material. High capacity 203 mAh g-1 can be maintained for 1000 cycles without any fade at a high rate of 300 mA g-1. Various metal/carbon nanocomposites synthesized by high energy ball milling were also evaluated as sodium ion battery anodes. SnSb/C nanocomposite material exhibits the best electrochemical performance. A close look into the reaction mechanism shows that SnSb reacts with Na as one reaction (xNa + SnSb ↔ NaxSnSb) rather than two separated sodium-tin and sodium-antimony reactions. The amorphous characteristic during the entire sodiation/desodiation process observed by in-situ XRD is believed to be helpful for accommodating the volume expansion and thus leads to good capacity retention. SnSb_C/ NaNi1/3Mn1/3Fe1/3O2 full cell was also demonstrated with decent capacity (75 mAh g-1), high voltage (3 V) and also good capacity retention. PVD SnSb thin films deposited on copper substrate with different Sn/Sb ratio were also tested in sodium ion batteries. Stable capacity of 450 mAh g-1 can be achieved for SnSb thin film for 50 cycles with the presence of FEC electrolyte additive. These thin film anode materials have ultrahigh volumetric energy density since both carbon and binder are absent in those electrodes. Multiple conversion type anodes are also evaluated. Among them, SnO and Sb2O3 show the best performance. These two materials react with sodium via a two-step reaction (first step, conversion reaction; second step, alloy reaction). 500 mAh g-1 and 550 mAh g-1 capacity can be obtained from SnO/C and Sb2O3/C composites, respectively. Alloy reaction has much faster kinetics compared to conversion reaction. As a result, SnOx or SbOx could be coexisting with some NaxSn or SbxSn alloys. On the other hand, inferior performance is achieved for pure conversion type anodes such as Co3O4, Fe2O3, Fe3O4 and so on. Finer particles and morphology engineering could be critical in order to obtain decent electrochemical performance from these oxides as anode materials for sodium ion batteries