Figure 1.4Potential profile of an NMC111 cathode, a graphite anode and the resulting cell potential during the first battery charge (C/10 rate). The electrode potentials are measured against an internal lithium reference electrode. The areas shaded in blue and red color indicate regions of cathodic and anodic electrolyte instability.
Figure 1.4 shows typical anode and cathode potential profiles and the resulting cell potential during the charge of a graphite/NMC111 cell. The specific energy of the cell is the area under the black curve (spec. energy [Wh kg-1] = voltage [V] x spec. ca-pacity [Ah kg-1]). There are two general possibilites for increasing the specific energy, i) increase the cell potential and ii) increase the specific capacity. The only option for increasing the cell potential is to increase the working potential of the cathode as the graphite anode already operates very close to the lower boundary of the lithium metal potential. The specific cell capacity is currently limited by the cathode mate-rials, with achievable specific capacites between 120 and 180 mAh g-1 (see Table 1.1) in comparison to 372 mAh g-1 for graphite. Therefore, in the short and mid term, the specific energy of state of the art lithium-ion batteries can gradually be improved by closing the gap between theoretical and practical specific capacity for NMC based cathode materials (see Table 1.1). This can be achieved by increasing the charging cut-off potential as shown in Figure 1.4. It is possible to entirely delithiate NMC111
if the material is charged to 5.0 V vs. Li/Li+.[44] The close to linear capacity increase at potentials above 4.2 V vs. Li/Li+ leads to an almost quadratic increase in spe-cific energy upon increasing the charging cut-off potential.[45] Unfortunately, cut-off potentials of higher than 4.4 V (vs. graphite) severely reduce battery life because the cathode potential exceeds the stability limit of the battery electrolyte (see Figure 1.4), causing electrochemical electrolyte oxidation which is accompanied by massive transi-tion metal dissolutransi-tion from the cathode active material.[45;46;47] The latter is probably caused by acidic etching due to protic electrolyte oxidation products.[48;49] Transition metal ions can diffuse through the electrolyte and precipitate on the graphite anode where they inhibit the passivating function of the SEI and catalyze electrolyte reduc-tion. While the exact mechanism of this transition metal triggered side reaction on the graphite electrode is still heavily debated, this process is considered to be the main reason for the fast capacity fading of NMC cells cycled to high cut-off poten-tials.[45;46] In addition to the electrochemical electrolyte oxidation, the NMC cathode material itself is also chemically unstable at high delithiation degrees (x < 0.2 in LixMO2). Our group recently reported that both normal NMC and lithium and man-ganese rich layered ”high-energy NMC” (HE-NMC) irreversibly release oxygen from the particle surface according to MO2 −−→MO +12O2 which is also believed to further contribute to electrolyte decomposition.[50;51] As a consequence, the choice of upper cut-off potential for graphite/NMC cells is a compromise between improving the spe-cific capacity while retaining a sufficient battery life. Alternatively, the utilization of the NMC cathode material can also be improved by increasing the nickel content. For the same cut-off potential of 4.3 V vs. Li/Li+ (about 4.2 V cell potential), NMC811 shows a reversible capacity of 200 mAh g-1 and a specific capacity of 760 Wh kg-1 which compares to 160 mAh g-1 and 600 Wh kg-1 for NMC111.[10;52] As a downside, the propensity for oxygen release also scales with the nickel content, which can be rationalized by the similarity in composition of NMC811 (LiNi0.8Mn0.1Co0.1O2) and NCA (LiNi0.8Co0.15Al0.05O2) which is well known for facile oxygen release.[51]. In ad-dition to improving NMC, entirely new cathode materials offering higher potentials and/or specific capacites are currently beeing explored. Possible candidates are the LiNi0.5Mn1.5O2 spinel[53] with an average potential of 4.7 V and a capacity of about 120 to 140 mAh g-1 or the above mentioned high-energy NMC[54;55] with a specific capacity of about 250 mAh g-1.[10]
On the anode side, the addition of small quantities of novel anode active materials like silicon (3600 mAh g-1for 15 Li++ 15 e–+ 4 Si−−→Li15Si4)[56]to a graphite electrode is
Figure 1.5Scheme of undesired side reactions in lithium-ion batteries which are investigated within this PhD project.
already used for improving the specific capacity.[7;57]The use of high silicon contents, or even pure silicon, is hampered by its massive volume expansion and contraction upon lithium intercalation and deintercalation of up to 300%, causing particle cracking and ongoing SEI formation. Various mitigation strategies like nano-structuring have been investigated but so far no reversible silicon anode is available.[58]
In the long term, lithium metal would be the ideal anode material due to its very high specific capacity of 3860 mAh g-1. Unfortunately, the lithium plating process (Li++ e– −−→Li) does not occur homogeneously over the electrode but forms micro-structured dendrites.[59]It is generally accepted that dendrite formation increases with the geometric plating current density (in mA cm-2) but, despite more than 30 years of intensive research, the fundamental mechanism of the process is still controver-sial.[60;61;62] Dendrite growth has been attributed to electric field inhomogeneities,[63]
lithium ion concentration gradients,[64;65] preferred lithium deposition at kinks/defect sites[66;67], or even mechanical stress within the electrode,[62] but so far no model can explain all experimentally observed dendrite growth patterns. In fact, dendrite growth has been observed to take place at the tip[62;68], in the middle,[67] and also at the base[68;69] of existing dendrites and to be either directed towards the counter
electrode or occur randomly and undirected.[67;69;70]
Approaches for the reduction or total suppression of lithium dendrite formation have mainly focused on electrolyte additives to improve the SEI properties and to achieve a more homogeneous current distribution or on strong mechanical barriers to prevent internal short circuits, but so far with limited success only.[59] The consequence of lithium dendrite formation is ongoing electrolyte and active lithium loss due to per-manent SEI renewal on the freshly exposed lithium metal surface. Also, during the stripping process, lithium metal can loose electronic contact to the electrode and form so-called ”dead lithium”. Furthermore, lithium dendrite formation is a serious safety hazard due to the above mentioned possibility of internal shorts. The consequences of lithium dendrite formation on lithium metal anodes and undesired lithium plating on graphite electrodes (see previous chapter) are similar, but the nature of these two processes is different. On graphite electrodes, lithium plating is an undesired side reaction which can be prevented by using proper charging conditions while lithium dendrite formation appears to be intrinsic to the (desired) lithium plating/stripping process on lithium metal anodes.
Figure 1.5 visualizes lithium-ion battery side reactions and their interconnections which are investigated in this PhD thesis. The diffusion of reaction products from one elec-trode to the other in combination with further side reactions is referred to as ”elecelec-trode crosstalk”.