Battery technologies for robots and drones – A brief introduction

Battery technologies are vital to delivering significant advances in a wide range of industries, from autonomous vehicles, robotics, and drones to renewable power. The majority of robots are required to be autonomous and free from an AC supply; therefore, they must rely wholly on batteries for their source of power.

They should be portable, compact, lightweight, maintenance-free, economical, rugged, resistant to abuse and safe in use and under accident conditions, stable to provide enough power over most of the discharge, and capable of recharging with a capacity of many charges and discharge cycles. This catapulted battery technology to the top of the priority list for many players, leading to a massive boom in investment, as companies try to build key positions in the market.

Following the invention of the rechargeable battery nearly 150 years ago, research has led to a wide range of technologies being used today. However, each technology has its strengths and weaknesses – product/technology designers, therefore, need to choose wisely for their particular applications.

Before we discuss each battery technology, it is essential to understand what a battery pack and a battery cell consist of. A battery pack contains battery cells (as you find in a TV remote control) and a battery management system, which regulates. A battery cell, on the other hand, contains multiple components – electrolyte fluids and electrodes that differ in chemistry, yielding different battery characteristics.

Lead-acid batteries

Lead-acid battery is the grandfather of all rechargeable batteries. It was the first rechargeable batteries ever made. Although the technology is outdated, it has stood the test of time and is still among today’s most widely used types. It is popular due to its low cost of capital and ability to operate efficiently even at low temperatures, which often prevails over their low energy densities and low life cycle times.

There are two prominent lead-acid battery families. The flooded type has an optimal cost of capital, decreasing as low as $60/kWh for large systems. It is less than one-third of the current cost of the lithium batteries used in most EVs. However, it has a few downsides such as low cycle life, low charging rate, and maintenance requirements, in which the battery needs to be supplemented with water to remain “flooded.” The second family named sealed batteries applies a slightly more advanced design that does not require a water supply. This eliminates maintenance costs and increases the life cycle, but doubles the cost of capital.

Advantages:

  • Market-leading battery type
  • Available in large quantities
  • Available in varieties of sizes and designs
  • Relatively high efficiency
  • Low specific costs

Disadvantages:

  • Low life cycle
  • Limited energy density
  • Hydrogen evolution in some designs
  • High maintenance costs

Lithium-ion batteries

In the last decade, lithium-ion (Li-ion) batteries have gained tremendous attention. Although already commercialized in 1991, constant marginal cost and performance improvements over the past 25 years have unlocked an array of new applications, making battery-related breaking news a common sight. The rapid decline in costs is mainly due to two drivers at the root — first, the massive increase in scale across all steps of the manufacturing value chain. Second, the increase in cell performance, making cells cheaper on a cost/kWh basis.

The constant search for powerful battery components has now led to a full breed of Li-ion battery compositions. While a perfect battery remains a work in progress, different variants of the battery’s three main components (anode, cathode, and electrolyte system) lead to some strengths and weaknesses. In current systems, for example, the cathode limits the power, while the anode limits the charging.

Advantages:

  • High energy density
  • High power to capacity ratio
  • Little or no maintenance
  • Low self-discharge
  • High energy efficiency
  • Long calendar lifetimes
  • A large number of cycles

Disadvantages:

  • Safety – the need for protective circuit
  • High initial costs
  • Thermal runaway possible when overcharged or crushed

Their cathode chemistry commonly classifies current Li-ion batteries. Five solutions are currently available:

LCO (lithium cobalt oxide)

The most mature cathode chemistry is LCO (lithium cobalt oxide), which has made Li-ion commercialization possible. It produces cells with the highest density of volumetric energy, but with a downside of low power density and low capacity for cycling. Cost is proving to be an increasingly significant issue since the cathode is made entirely of cobalt. The current efforts to innovate are focused on squeezing the last drops out of the battery’s performance by increasing the material’s voltage and energy capacity. Unless there is a better alternative, this technology will remain the cathode of choice in consumer electronics because of two reasons: it has the highest volumetric energy density, and, in these applications, the willingness to pay is generally higher.

LFP (lithium-iron-phosphate)

The LFP batteries (lithium-iron-phosphate) take a different approach. The cathode consists of more abundant iron and phosphate, resulting in a lower cost of raw material. However, cells produced with LFP have low energy density due to the low voltage and low energy capacity inherent in LFP, which eventually makes it a more expensive battery when measured on a cost / kWh basis. Due to its rigid olivine structure, the cathode material is still favored, which gives the material its extremely high power and long life cycle.

This technology is already very close to its maximum theoretical performance, giving little room, apart from cost-cutting, to further improvements. The Chinese battery industry has grown dramatically along the cheap LFP production path of using rotary kilns. Since other technologies are evolving, higher-performance materials are today gradually replacing LFP in applications like EVs, leaving the market inundated with cheap LFP overcapacity. In contrast, high-performance LFP, commonly produced through hydrothermal methods, will maintain a strong position in high-power (e.g., HEVs and power tools) or high-cycle life (CEVs, grid storage) applications.

NCA (lithium nickel cobalt aluminum oxide)

NCA (lithium nickel cobalt aluminum oxide) is a cathode material with high energy content. The current focus is to increase the nickel content further, resulting in higher energy density while at the same time reducing cobalt use, effectively reducing the cost / kWh in two ways. Tesla mainly uses NCA, whereas NCM is used by all other EV makers. That dates back to the first time Tesla produced its Roadster (2005). A cheap, high-energy-density cell was needed, and NCA was the only option at the time, as NCM would not be commercialized until 2009. Tesla will most likely continue to use NCA in its current development cycle, as it is accustomed to using it in Panasonic’s supplied cylindrical cell format. However, for energy-storage applications, Tesla has already switched to NCM, suggesting that a future switch for EVs could take place soon.

NCM (lithium nickel cobalt manganese oxide)

NCM (lithium nickel cobalt manganese oxide) is a diverse material that depends on the stoichiometric balance between nickel, cobalt, and manganese. An even ratio (called NCM 1-1-1) is best suitable for high-power applications, whereas higher nickel contents (5-3-2 or 6-2-2) provide higher energy density while at the same time reducing cobalt dependence. These are two critical reasons the industry is attempting to market the 8-1-1 nickel-rich NCM–major producers expected to have the first solutions to market in early 2018. NCM remains the cathode material of choice for almost all EV manufacturers (apart from Tesla) until the use of superior 5V cathode materials. Even then, due to the long and conservative development cycles of the automobile industry, NCM will continue to be used for another 5-7 years. In other applications, NCM will also be the occasional choice, such as energy storage, HEVs, and e-buses.

LMO (lithium manganese oxide)

LMO (lithium manganese oxide) is similar to LFP because it can deliver high power and lacks energy density, but is 2 to 3 times cheaper. The main problem preventing its mass adoption is its low stability, as shown by Nissan’s recent shift away from using the technology due to continuing battery malfunctions.

Key battery performance indicators

Battery performance indicators identify key calculations required to evaluate rechargeable battery systems properly. They make the assessment of a battery system based on parameters such as the energy stored per mass (specific energy) or volume (energy density) of a cell, power (energy per time and volume or weight), lifetime, environmental friendliness, safety and costs per stored energy. You can see some of the key battery performance indicators below:

  • Capital cost (EUR/kWh) – the upfront cost to buy a battery (excluding O&M)
  • Safety – Resistivity against thermal runaway
  • Cycle lifetime – It is the number of cycles a battery can be discharged from 100% to 20% until capacity fades to 80% of its original capacity.
  • The energy density (Wh/kg or Wh/L) – Amount of energy which a battery can hold, measured by weight or volume
  • Power density (C-rate) – Rate at which a battery is discharged relative to its maximum capacity
  • Charging time (C-rate) – It is the rate at which a battery is charged relative to its maximum capacity.
  • Reliability – It is the ability to operate in low temperatures or extreme conditions
  • Others – Other properties, such as maintenance costs, shelf lifetime, self-discharge, or charging efficiency

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