Lithium polymers, separator

In battery applications, new hthium ion batteries called lithium ion polymer batteries (or more simply but misleadingly, lithium polymer batteries) work with a full matrix of ionically conducting polymer, this polymer being present inside the porous electrodes and as a separator between the electrodes. They are offered in attractive flat shapes for mobile applications (mobile phones, notebooks). 

Lithium polymer electrolytes formed by dissolving a lithium salt LiX (where X is preferably a large soft anion) in poly(ethylene oxide) PEO can find useful application as separators in lithium rechargeable polymer batteries.Thin films must be used due to the relatively high ionic resistivity of these polymers. For example, the lithium-ion conductivity of PEO—Li salt complexes at 100 °C is still only about Viooth the conductivity of a typical aqueous solution. 

The lithium polymer battery (LPB), shown schematically in Fig. 7.21, is an all-solid-state system which in its most common form combines a lithium ion conducting polymer separator with two lithium-reversible electrodes. The key component of these LPBs is the polymer electrolyte and extensive work has been devoted to its development. A polymer electrolyte should have (1) a high ionic conductivity (2) a lithium ion transport number approaching unity (to avoid concentration polarization) (3) negligible electronic conductivity (4) high chemical and electrochemical stability with respect to the electrode materials (5) good mechanical stability (6) low cost and (7) a benign chemical composition. 

Jung, Y.S., Cavanagh, A.S., Gedvilas, L., Widjonarko, N.E., Scott, I.D., Lee, S.H., Kim, G.H., George, S.M., Dillon, A.C., 2012. Improved functionality of lithium-ion batteries enabled by atomic layer deposition on the porous microstructure of polymer separators and coating electrodes. Adv. Energy Mater. 2, 1022-1027. 

With the use of sohd to gelatinous, polymeric electrolyte layers that ensure electrical separation of the electrodes and the unity of the cell components combined with good conductivity for lithium ions, it is not necessary to employ hquid electrolytes in lithium batteries. This simplifies the production of lithium polymer batteries, which are also safer in operation because the electrolyte is polymeric, not liquid. This battery cell structure facilitates production of thin foil batteries, a favorable form for use in portable devices. The expected performance data correspond to what is obtained with other lithium ion systems. 

The discovery and the characterization of ionically conducting polymeric membranes (see Chapters 1 and 2) have provided the interesting possibility of developing new types of lithium batteries having a thin-layer, laminated structure. Various academic and industrial laboratories [1-5] are presently engaged in the development of this revolutionary type of battery, i.e. the so-called Lithium Polymer Battery (LPB). The key component of the LPB is the polymeric ionic membrane which acts both as electrolyte and separator furthermore, the membrane can be easily fabricated in the form of a thin film (typically 50 jum thickness) by a number of convenient casting techniques. 

In this case, we have an electrolyte identical to that which is present in lithium-polymer batteries, made of poly(ethylene oxide) (or PEO) in the presence of a lithium salt, solid at ambient temperature, and which needs to be heated above ambient temperature in order for the battery to work (T > 65°C for PEO). Thus, the electrolyte, in its molten state, exhibits sufficient ionic conductivity for the lithium ions to pass. This type of electrolyte can be used on its own (without a membrane) because it ensures physical separation of the positive and negative electrodes. This type of polymer electrolyte needs to be differentiated from gelled or plasticized electrolytes, wherein a polymer is mixed with a lithium salt but also with a solvent or a blend of organic solvents, and which function at ambient temperature. In the case of a Li-S battery, dry polymer membranes are often preferred because they present a genuine all solid state at ambient temperature, which helps limit the dissolution of the active material and therefore self-discharge. Similarly, in the molten state (viscous polymer), the diffusion of the species is slowed, and there is the hope of being able to contain the lithium polysulfides near to the positive electrode. In addition, this technology limits the formation of dendrites on the metal lithium... 

Initially, lithium polymer battery electrolytes were in the form of a dry sohd polymer electrolyte but it was found that performance could be improved by introducing gelled electrolyte into the separator system. Described by some as a gehonic electrolyte which is not a genuine polymer, these critics maintain that the batteries should be described as being plastic hthium ion and not hthium polymer. 

Positive and negative intercalation electrodes (lithium-ion) with a porous polymer separator layer filled with a liquid electrolyte (PVDF-based). 

Traditionally, lithium-ion separators were made from PE, PP, or some combination of the two, because these polyolefins provide excellent mechanical projjerties and chemical stability at a reasonable cost. Recently, ceramic materials and aramid polymers have been introduced as a means to improve the thermal stabihty of separators to temperatures of 200 °C and above. 

As discussed in previous chapters, the separators are an integral part of liquid electrolyte batteries including nonaqueous batteries such as lithium-ion, lithium-polymer, hthium-ion gel polymer, and aqueous batteries such as zinc-carbon, zinc-manganese oxide, lead-acid, nickel-based batteries, and zinc-based batteries. 

The polymer component in these batteries fulfills the function of a medium for ionic transport and a separator. The polymers are polyethers, PEO, or PPO. However, the lithium salts, dissolved in these polymers, have 100-fold lower conductivity than that of a lithium salt dissolved in water. The low conductivity below 70 °C, the reactivity of the interface with the lithium metal electrode, and the issues related to mechanical properties and electrochemical stability need to be resolved before the lithium polymer battery has acceptable performance. The use of inorganic composite membranes, described in a subsequent section, has been shown to result in improved ionic conductivity. 

Shutdown separators provide a level of protection from external shorting and overcharging in lithium-ion cells. PE separators have a shutdown temperature of 130 °C, at which temperature the impedance increases significantly. The impedance increase results from pore collapse that occurs at the softening point of the microporous polymer separator, and this can effectively stop ionic transport between... 

It is well known today that the SEI on both lithium and carbonaceous electrodes consists of many different materials including LiF, Li2C03, LiC02R, Li20, lithium alkoxides, nonconductive polymers, and more. These materials form simultaneously and precipitate on the electrode as a mosaic of microphases [5, 6], These phases may, under certain conditions, form separate layers, but in general it is more appropriate to treat them as het-eropolymicrophases. We believe that Fig. 13(a) is the most accurate representation of the SEI. 

The liquid electrolytes used in lithium batteries can be gelled by addition of a polymer [25] or fumed silica [26], or by cross linking of a dissolved monomer [271. Depending on the mechanical properties, gelled electrolytes can be used as separators, or supported by a conventional [27]... 

At this time the only commercially available all-solid-state cell is the lithium battery containing Lil as the electrolyte. Many types of solid lithium ion conductors including inorganic crystalline and glassy materials as well as polymer electrolytes have been proposed as separators in lithium batteries. These are described in the previous chapters. A suitable solid electrolyte for lithium batteries should have the properties... 

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