The main components of a lithium-ion battery are the active materials in the negative and positive electrode and the liquid electrolyte. According to common convention in the field of lithium-ion batteries, the negative electrode is called anode and the positive electrode is called cathode throughout this work, following the nomenclature of the discharge reaction. A typical lithium-ion battery contains graphite as anode active material which can reversibly intercalate lithium ions during battery charge and de-intercalate lithium ions during discharge (Li+ + e– + C6 −−−−−)charge−−−−−*
discharge LiC6). On the cathode side, a transition metal oxide like LiCoO2 is used from which lithium ions are extracted during charge and inserted during discharge (LiCoO2 −−−−−)−−−−−charge*
discharge Li1 –xCoO2+ xLi+ +xe–). The cell reaction for such a lithium-ion battery is simply the sum of the half cell reactions (LiCoO2 +xC6 −−−−−)−−−−−charge*
discharge Li1 –xCoO2 +xLiC6). This kind of cell chemistry is called intercalation chemistry because the active materials, also referred to as hosts, retain their general structure during lithium intercalation and
Figure 1.1 Working principle and main components of a lithium-ion battery. Image from reference [11]. Reprinted with permission from AAAS.
deintercalation.1 It is also the host material which is getting oxidized or reduced during battery operation (e.g. Co3+ −−)−−* Co4+) while lithium ions shuttle between the electrodes, hence the alternative name ”rocking chair battery” for a lithium-ion battery. The ionic conductivity between the electrodes is provided by an aprotic electrolyte containing a lithium salt while a porous polyolefine separator prevents short circuiting.[20]In this section, the most important aspects regarding the basic cell components (anode/cathode active materials and electrolyte) are briefly reviewed.
Graphite is the standard anode active material because it offers a high specific capac-ity of 372 mAh g-1 and a low and flat working potential of 100 to 200 mV vs. Li/Li+. The characteristic potential profile of graphite can be assigned to the well known stag-ing behavior.[21] In Li0.5C6 (stage II) two graphene sheets separate each lithium layer whereas in Li1.0C6 (stage I) the graphene and lithium layers alternate.[22] For lower intercalation degrees (x <0.5 in LixC6), the mechanism of lithium intercalation is still controversial.[23] The magnetic and electronic properties of the graphite intercalation compounds change drastically upon lithium intercalation, for example the c-axis con-ductivity (perpendicular to graphene layers) is three orders of magnitude higher in LiC6 in comparison to pristine graphite.[22;24;25;26] The potential of the lithium inter-calation into graphite is well below the thermodynamic stability limit of all aprotic electrolyte solvents. Therefore, stable cycling of a graphite electrode is only possible because of the so-called solid electrolyte interphase (SEI), a concept first introduced by Emanuel Peled in 1979.[27] The SEI is a several nano meter thick passivation layer which forms in the very first charge and consists of inorganic and organic electrolyte reduction products like lithium fluoride, lithium carbonate and lithium ethylene di-carbonate (LEDC);[28] it prohibits ongoing electrolyte reduction as it is electronically insulating but allows lithium ion diffusion, thereby enabling reversible lithium inter-calation and deinterinter-calation in graphite.[29;30] In a state of the art lithium-ion battery, the limitation of the charging current arises at the graphite electrode.[17] The low working potential of the graphite electrode (100 to 200 mV above Li/Li+ potential) is advantageous in terms of cell potential and specific energy, but poses the risk of undesired lithium metal plating upon battery charge. At high charging currents, the cell polarization can drive the potential of the graphite electrode below the Li/Li+ po-tential and plating of metallic lithium becomes thermodynamically possible. Lithium plating has to be prevented as it causes irreversible capacity loss and also poses a
1The term ”intercalation” was first used in the 1950s for chemical intercalation into graphite.[19]
Within the battery community the term ”intercalation chemistry” was introduced by S. Whittingham in 1976.[2]
serious safety hazard, as discussed in more detail in the next chapter.[31]
The spinel type lithium titanate (LTO) is an alternative anode active material with a specific capacity of 175 mAh g-1and working potential of 1.55 V vs. Li/Li+.[32]On the one hand, the comparably high potential makes LTO an interesting anode material for cells with a high charging rate capability, as the above mentioned lithium plating side reaction is not taking place.[33] On the other hand, the high working potential significantly limits the cell potential and therefore the specific energy. There have been attempts to use novel high voltage cathode materials to achieve reasonable cell potentials also in combination with an LTO anode,[33;34]but in the end LTO anodes will probably be excluded from cells which are designed for high specific energy densities.
Hard carbons are another interesting group of anode active materials. In general, the term hard carbon refers to disordered carbonaceous materials which cannot be graphi-tized at high temperatures. Depending on the preparation method, hard carbons have specific capacities ranging from 200 to almost 1000 mAh g-1.[35]Hard carbons exhibit a sloped potential profile which is intermediate between graphite and LTO,[8;35] making them promising anode materials for applications which require fast charging and a high specific energy. Hard carbons were used in Sony’s second generation lithium-ion battery in 1992 before being replaced by graphite which offers a higher energy density (in Wh L-1).[1] In the near future, the necessity for higher charging rates might cause a revival of hard carbons as anode materials.
Figure 1.2 Annual global production of LCO, NMC, LMO, LFP and NCA cathode materials for the years 2014 and 2015. Note that the NMC value includes materials of different Ni, Co and Mn content. Data provided by Christophe Pillot.[36]
On the cathode side, several different active materials are commonly used in state of the art lithium-ion batteries; Figure 1.2 shows the yearly global production for the five most common cathode materials for the years 2014 and 2015. Most cathode materials are lithium containing transition metal oxides. There is a class of layered transition metal oxides including LiCoO2 (LCO), LiNi0.8Co0.15Al0.05O2 (NCA) and LiNixMnyCozO2 (NMC, with x + y + z = 1) which all have anα-NaFeO2– type struc-ture and there are also a few spinel materials like LiMn2O4.[37] For NMC, the atomic ratio of the transition metals Ni, Mn and Co can be varied and is indicated by sub-scripts (e.g. NMC111 = LiNi1/3Mn1/3Co1/3O2). Currently, NMC111 is the most com-monly used material, but other stochiometries like NMC532, NMC622 and NMC811 are under development (see next section). The only commonly used non-oxide cathode material is the phospho-olivine LiFePO4(LFP) which was first described in 1997 by the Goodenough group.[38] While the different cathode materials all have specific advan-tages and disadvanadvan-tages, one can make a simple comparison of the specific energy and intrinsic material safety as shown in Table 1.1. For all cathode materials except LFP, the practically achievable specific capacity is lower than the theoretical capacity if the entire lithium could be extracted during charge (x <1 in LiCoO2
charge
−−−→Li1 –xCoO2+ xLi+ + xe–). The amount of lithium which is extracted during battery charge is controlled by the upper cut-off potential. NCA delivers the highest practical specific energy of 750 Wh kg-1, followed by LCO and NMC111 with around 600 Wh kg-1. LMO and LFP give lower practical specific energies of 500 and 450 Wh kg-1 which is due to their lower capacity and potential, respectively. The intrinsic safety characteristics of a cathode material improve with the onset temperature of oxygen release. Once oxygen is released at elevated temperatures, it can cause the ignition of the electrolyte and therefore contribute to a so-called thermal runaway.[39] According to Table 1.1, the layered oxidizes LCO, NCA and NMC can release oxygen at relatively low tem-peratures of 150, 160 and 210 ◦C, respectively. In contrast, the spinel LMO has as significantly higher onset temperature of 265 ◦C, while the phospho-olivine LFP does not release oxygen at all in the relevant temperature range because of the covalent oxygen bonding within the phosphate anion. In general, for cathode materials, spe-cific energy and intrinsic material safety show opposing trends. Except for LCO, all cathode materials listed in Table 1.1 are currently used in battery electric vehicles (BEV) by major car producers.[7;10]
A standard electrolyte in a lithium-ion battery contains a mixture of cyclic and linear alkyl carbonates as solvents and LiPF6 as conducting salt. The cyclic alkyl
carbon-Table 1.1 Selected electrochemical and chemical properties of lithium-ion cathode materials. The first specific capacity is the practically achievable value, the one in brackets is the theoretical value.
The average potential is the midpoint voltage at a C/20 rate. LFP does not show any oxygen release.
Spec. energy[10] Spec. capacity[7] Average potential[7] Oxygen release[39]
[Wh kg-1 ] [mAh g-1] [V vs. Li/Li+] [◦C]
LCO 600 155 (274) 3.9 150
NMC 600 160 (278) 3.8 210
NCA 750 180 (279) 3.7 160
LMO 500 100-120 (148) 4.0 265
LFP 410 160 (170) 3.4
-ates enable the dissolution of the conducting salt due to their high dielectric constants, while the linear carbonates decrease the solvent viscosity to optimize the ionic conduc-tivity.[40] The most commonly used cyclic carbonate is ethylene carbonate (EC), for the linear carbonates dimethyl carbonate (DMC) or ethyl methyl carbonate (EMC) are typically used. For the last 25 years, ethylene carbonate has been an indispensable electrolyte component as it forms an SEI which prevents solvent co-intercalation and resulting graphite exfoliation.[8] LiPF6 is used as standard conducting salt as it offers good overall properties and especially because it passivates the aluminum current col-lector. The latter is not the case for most other conducting salts like, for example, bis(trifluoromethane)sulfonimide lithium salt (LiTFSI).[41]In order to improve the cy-cle and calendric life of lithium-ion batteries, small quantities of so-called electrolyte additives are used. Most electrolyte additives are reduced on the graphite electrode within the first charge and form a more stable and passivating SEI.[42] In commercial cells the most common electrolyte additives are vinylene carbonate (VC) and prop-1-ene-1,3 sultone (PES).[43] The structures of these electrolyte components are listed in Figure 1.3.
Figure 1.3 Structures of most common electrolyte solvents, conducting salts and additives.