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Reel-to-Reel Manufacture of High Energy Density, All-Solid-State Lithium-Ion CellsBy Professor Ian M Ward University of Leeds, UK

The Challenge – Improved Safety at Lower Cost

A well-publicised recent spate of fires involving

electric vehicles in the US has once again

opened the debate on the safety of lithium-ion

batteries as sources of automotive power.

Whether or not large-scale batteries are less

safe than tanks of highly flammable petroleum

products is beyond the scope of this article.

However, the fact remains that question marks

still hang over lithium-ion batteries not only in

terms of safety but in terms of cost and

performance. Such issues are being

addressed at the University of Leeds by a

team headed by Professor Ian Ward FRS. The

basis of the work is the development of a new

class of materials that offers an approach to

improved safety, maximum performance and

high speed cost effective manufacturing of

lithium-ion cells.

Conventional lithium-ion batteries use solvent

based liquid electrolytes with a porous

mechanical separator to hold the electrodes

apart. Individual cells are effectively sealed

containers that are filled with liquid in a

manufacturing process that is cumbersome

and often hazardous; the solvents employed

are highly toxic and highly flammable. Besides

the obvious environmental issues surrounding

the large scale use of organic solvents,

numerous instances of large scale fires within

production plants have been reported.

Historically there are also many well

documented cases of fires and explosions

involving lithium-ion batteries resulting from

mechanical damage, short circuits and

overheating within the cells.

Recent efforts to address safety issues, such

as chemical electrolyte additives and novel

separator materials, have done nothing to

address the underlying issues.

The Innovation

The team at Leeds has been working on two

inter-related, patented technologies – a solid

state polymer gel electrolyte and a high speed

extrusion/lamination, reel-to-reel

manufacturing process.

Polymer gel electrolytes based on

polyvinylidene fluoride (PVDF) and lithium

salts were invented at Leeds University as part

of a major project in the IRC in Polymer

Science and Technology. The technological

breakthrough has been to produce semi-rigid

films with very high levels of ionic conductivity

- in the region of 10-3 Scm-1, comparable to

conventional liquid electrolytes.

The solid-state flexible gel creates a dry

system eliminating flammable solvents in the

finished product and the need for a separator.

Furthermore, being highly compliant, the

electrolyte film is ideal for use with high

swelling advanced electrode materials.

Besides the obvious safety aspects associated

with a dry system, elimination of the separator

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can account for significant material cost

reductions.

Figure 1: Cells based on gel electrolytes are all-

solid-state, thin, flexible and safe.

The robust, semi-flexible laminate can be

cut and packaged into a wide range of shapes,

such as flat wide sheets, folded to suit device

geometries or stacked to facilitate large scale

multi-cell batteries. Typical overall cell

thickness is around 0.3 – 0.4 mm. No

separator is required as the gel “micro-

encapsulates” the electrolyte to produce a dry

“pouch” cell.

Cathode

AnodeElectrolyte

OtherFoils

Separator

Figure 2: Conventional lithium battery cost

structure. The separator is a major cost component.

Unlike conventional cells, pouch cells vent at

low pressure, before thermal runaway, with

minimal electrolyte escape on venting or

puncture. The thin, flat shapes provide

excellent heat transfer and as the electrodes

are not subjected to high compressive forces

there is less likelihood of shorting.

Figure 3: Pouch cells are thin, flexible and can be

produced in a wide range of sizes and capacities.

Cell Manufacture

A patented extrusion/lamination process has

also been developed for the fabrication of cells

in which the solid polymer gel electrolyte is

simultaneously coated by both anode and

cathode in a fully automated reel-to-reel

process.

Figure 4. A pilot line has been constructed to

produce cells of high specific capacity and high

energy density for rechargeable lithium batteries.

The process is high speed and fully automated

offering the cost savings and repeatability

associated with continuous processes.

Continuous high speed production requires no

separator, eliminates complex mixing and

drying operations and readily allows cutting,

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stacking and packaging of large format cells.

Variable geometry permits a wide range of cell

size and capacity. Combined with the solid

state gel electrolyte the process offers

significant savings in manufacturing costs

when compared with the assembly of

conventional liquid electrolyte cells. The pilot

line developed in Leeds can run at 10 m/min

with a projected speed of 100 m/min for

commercial scale equipment. The process is

fully scalable allowing the high speed

manufacture of wide format cells tailored to

high capacity rechargeable battery packs.

Figure 5. Laminated electrodes. The extrusion

lamination process enables high speed production

of thin polymer batteries with an estimated 1/3 less

capital investment and up to 25% lower material

costs compared with cylindrical battery cells.

Compatibility and Performance

Cell capacity tests comparing the polymer gel

electrolyte with conventional separators have

shown that Leeds cells typically have equal or

higher energy than conventional cells.

However, in any lithium battery system cell

performance is governed by the electrodes,

specifically the number of lithium ions that can

be stored and transported in a given volume.

In order to meet demands for higher energy

densities, an essential requirement if electric

vehicles are to become mainstream, new

chemistries are being developed such as

silicon alloy nanocomposites. A part of the

Leeds program has been to demonstrate the

compatibility of the gel electrolyte with a wide

range of electrode materials including:

Conventional carbon anodes with

LCO, NCA and LFP cathodes

BASF’s next generation, high

performance HE NCM cathode

3M’s high energy Si alloy anode

Figure 6. Projected cell energy densities based on

Leeds gel electrolyte with various anodes versus

BASF HE NCM in 5-12 amp-hour pouch cells.

In a recent development program, in

collaboration with PolyStor Energy Corp and

funded by DARPA, cells have been built and

tested using Leeds gel electrolyte with BASF

HE NCM and 3M SiA electrode materials

which enabled the construction of a BB-2590

battery – a military standard battery used in a

wide range of field operations including

communications and robotics. The battery was

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independently tested and shown to have a

capacity of 600 watt-hours. Conventional BB-

2590’s are < 200 watt-hours.

Proposition Position Today

Both the gel electrolyte and the

extrusion/lamination technology have granted

patents. The Leeds team is responding to

industry drivers in both product development &

customisation of the manufacturing process

with a number of developing collaborations

and alliances in the UK and overseas.

This publication contains general information and, although SMMT endeavours to ensure that the content is accurate and up-to-date at the date of publication, no representation or warranty, express or implied, is made as to its accuracy or completeness and therefore the information in this publication should not be relied upon. Readers should always seek appropriate advice from a suitably qualified expert before taking, or refraining from taking, any action. The contents of this publication should not be construed as advice or guidance and SMMT disclaims liability for any loss, howsoever caused, arising directly or indirectly from reliance on the information in this publication.

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