Lithium-ion separators

Figure 3. Generalized process for lithium-ion separator manufacturing. Each step of the separator manufacturing process has online detection systems to monitor the quality of the separator.

An emerging new group of lithium-ion separator comprises of inorganic composite membranes ( ceramic separators ) with their excellent wettability and exceptional thermal stability properties. Most of ceramic separators are porous mats made of ultrafine inorganic particles bonded using a small amount of binder. 

In the current era lithium ion separators have gone through a similar engineering trajectory, and most systems now are either PE, PP, PvDF, polyolefin, or composites thereof with [5, 8, 9], engineered to be resistant to chemical attack by aprotic electrolytes. For alkaline systems, PP and PVA are analogous systems. Additionally, the low cost and abundance of both cellophane and cellulose make it an attractive choice of separator for low-cost zinc-alkaline primary cells. 

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. 

The poor efficiencies of coal-fired power plants in 1896 (2.6 percent on average compared with over forty percent one hundred years later) prompted W. W. Jacques to invent the high temperature (500°C to 600°C [900°F to 1100°F]) fuel cell, and then build a lOO-cell battery to produce electricity from coal combustion. The battery operated intermittently for six months, but with diminishing performance, the carbon dioxide generated and present in the air reacted with and consumed its molten potassium hydroxide electrolyte. In 1910, E. Bauer substituted molten salts (e.g., carbonates, silicates, and borates) and used molten silver as the oxygen electrode. Numerous molten salt batteiy systems have since evolved to handle peak loads in electric power plants, and for electric vehicle propulsion. Of particular note is the sodium and nickel chloride couple in a molten chloroalumi-nate salt electrolyte for electric vehicle propulsion. One special feature is the use of a semi-permeable aluminum oxide ceramic separator to prevent lithium ions from diffusing to the sodium electrode, but still allow the opposing flow of sodium ions. 

This section reviews the state-of-the-art in battery separator technology for lithium-ion cells, with a focus on separators for spirally wound batteries in particular, button cells are not considered. 

Note that a review of battery separators for lithium-ion cells was recently published [1] in Japanese. 

Currently, all commercially available, spirally wound lithium-ion cells use microporous polyolefin separators. In particular, separators are made from polyethylene, polypropylene, or some combination of the two. Polyolefins provide excellent mechanical properties and chemical stability at a reasonable cost. A number of manufacturers produce microporous polyolefin separators (Table 1.)... 

Table l. Commercially available microporous membrane materials used as separators in lithium-ion batteries. 

Short-circuit tests with lithium-ion batteries have been reported recently [35]. This work shows that the separator provides shutdown when the battery is subjected to an external short circuit with the PTC bypassed. The large increase in impedance of the separator is attributed to the temperature rise in the battery. 

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 iodide pacemaker batteries use lithum iodide as the electrolyte, separating the lithium anode and the iodine anode. The function of the electrolyte is to transport ions but not electrons. Lithium iodide achieves this by the transport of Li+ ions from the anode to the cathode. This transport is made possible by the presence of Li vacancies that are generated by the intrinsic Schottky defect population present in the solid. Lithium ions jump from vacancy to vacancy during battery operation. 

Figure 2. Lithium isotope separation effected during ion exchange with synthetic zeolite (Taylor and Urey 1938). As has been demonstrated repeatedly since that study, in both natural and synthetic experiments, Li is fixed more effectively in the exchanger than Li.

Oi T, Odagiri T, Nomura M (1997) Extraction of lithium from GSJ rock reference samples and determination of their lithium isotopic compositions. Anal Chim Acta 340 221-225 Oi T, Shimizu K, Tayama S, Matsuno Y, Hosoe M (1999) Cubic antimonic acid as column-packing material for chromatographic lithium isotope separation. Sep Sci Tech 34 805-816 Olsher U, Izatt RM, Bradshaw JS, Dailey NK (1991) Coordination chemistry of lithium ion a crystal and molecular structure review. Chem Rev 91 137-164... 

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... 

A battery is a transducer that converts chemical energy into electrical energy and vice versa. It contains an anode, a cathode, and an electrolyte. The anode, in the case of a lithium battery, is the source of lithium ions. The cathode is the sink for the lithium ions and is chosen to optimize a number of parameters, discussed below. The electrolyte provides for the separation of ionic transport and electronic transport, and in a perfect battery the lithium ion transport number will be unity in the electrolyte. The cell potential is determined by the difference between the chemical potential of the lithium in the anode and cathode, AG = —EF. 

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